Add a description for installing GraphicsMagick or ImageMagick in docs/README.md

[HyunSangHan]

Description of the Change

To try to contribute to Mocha docs, I proceeded with the installation as described in README.md of docs directory. However, when I tried to use npm start docs after installing, the following error occurred:

nps is executing `docs.postbuild` : buildProduction docs/_site/index.html --outroot docs/_dist --canonicalroot https://mochajs.org/ --optimizeimages --svgo --inlinehtmlimage 9400 --inlinehtmlscript 0 --asyncscripts && cp docs/_headers docs/_dist/_headers && node scripts/netlify-headers.js >> docs/_dist/_headers
Guessing --root from input files: file:///Users/temp/Downloads/exercise2/mocha/docs/_site/
 ✔ 0.006 secs: logEvents
 ✔ 1.320 secs: loadAssets
 ✔ 0.008 secs: bundleWebpack
 ✔ 0.063 secs: populate
 ✔ 0.015 secs: bundleSystemJs
 ✔ 0.057 secs: bundleRequireJs
 ✔ 12.659 secs: populate
 ✔ 0.096 secs: populate
 ✔ 3.664 secs: checkIncompatibleTypes
 ✔ 0.020 secs: removeRelations
 ✔ 1.916 secs: populate
 ✔ 1.895 secs: populate
 ✔ 1.249 secs: removeRelations
 ✔ 1.766 secs: addDataVersionAttributeToHtmlElement
 ✔ 1.488 secs: stripDebug
 ✔ 2.555 secs: externalizeRelations
 ✔ 0.004 secs: mergeIdenticalAssets
objc[45255]: Class GNotificationCenterDelegate is implemented in both /Users/temp/Downloads/exercise2/mocha/node_modules/canvas/build/Release/libgio-2.0.0.dylib (0x105f31578) and /Users/temp/Downloads/exercise2/mocha/node_modules/sharp/vendor/lib/libgio-2.0.0.dylib (0x107bdf498). One of the two will be used. Which one is undefined.
 ✔ 0.274 secs: processImages
 ✔ 1.484 secs: spriteBackgroundImages
(node:45255) DeprecationWarning: max() is deprecated, use resize({ fit: "inside" }) instead
(node:45255) DeprecationWarning: withoutEnlargement() is deprecated, use resize({ withoutEnlargement: true }) instead
 ⚠ WARN: docs/_site/images/matomo-logo.png: Error executing Stream: The gm stream ended without emitting any data
(node:45255) UnhandledPromiseRejectionWarning: Error: docs/_site/images/matomo-logo.png: Error executing Stream: The gm stream ended without emitting any data
    at Socket.stdout.on.once (/Users/temp/Downloads/exercise2/mocha/node_modules/express-processimage/lib/getFilterInfosAndTargetContentTypeFromQueryString.js:821:31)
    at Object.onceWrapper (events.js:286:20)
    at Socket.emit (events.js:203:15)
    at endReadableNT (_stream_readable.js:1145:12)
    at process._tickCallback (internal/process/next_tick.js:63:19)
(node:45255) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:45255) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
cp: docs/_dist/_headers: No such file or directory
The script called "docs.postbuild" which runs "buildProduction docs/_site/index.html --outroot docs/_dist --canonicalroot https://mochajs.org/ --optimizeimages --svgo --inlinehtmlimage 9400 --inlinehtmlscript 0 --asyncscripts && cp docs/_headers docs/_dist/_headers && node scripts/netlify-headers.js >> docs/_dist/_headers" failed with exit code 1 https://github.com/kentcdodds/nps/blob/v5.9.5/other/ERRORS_AND_WARNINGS.md#failed-with-exit-code
The script called "docs" which runs "nps docs.prebuild && nps docs.api && eleventy && nps docs.linkcheck && nps docs.postbuild" failed with exit code 1 https://github.com/kentcdodds/nps/blob/v5.9.5/other/ERRORS_AND_WARNINGS.md#failed-with-exit-code
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! mocha@6.2.0 start: `nps "docs"`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the mocha@6.2.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/temp/.npm/_logs/2019-10-03T08_45_45_219Z-debug.log

After searching, I installed GraphicsMagick and finally I was able to make it work. It is understood that GraphicsMagick must be installed for execution, but I believe there is not enough explanation about it. Therefore, I think some additional description of this case can be more helpful for beginners(like me!). I propose to add the description as follows:

• If you get the error message below when running npm start docs, follow this guide to install GraphicsMagick.

⚠ WARN: docs/_site/images/matomo-logo.png: Error executing Stream: The gm stream ended without emitting any data
(node:45255) UnhandledPromiseRejectionWarning: Error: docs/_site/images/matomo-logo.png: Error executing Stream: The gm stream ended without emitting any data
    at Socket.stdout.on.once (/Users/username/mocha/node_modules/express-processimage/lib/getFilterInfosAndTargetContentTypeFromQueryString.js:821:31)
    at Object.onceWrapper (events.js:286:20)
    at Socket.emit (events.js:203:15)
    at endReadableNT (_stream_readable.js:1145:12)
    at process._tickCallback (internal/process/next_tick.js:63:19)
(node:45255) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:45255) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
cp: docs/_dist/_headers: No such file or directory

Benefits

I believe it can be more helpful for beginners.

[outsideris]

Great. I saw gm errors when I was building docs.
I talked about this, see https://gitter.im/mochajs/contributors?at=5d55623f7d56bc60805a72a8

[outsideris]

How about GraphicsMagick or ImageMagick. I think both are work.

[HyunSangHan]

Thank you for your opinion! I'll check and fix it. :-)