docs: npm home nyc goes to github master branch README

This causes confusion amongst users who think they're reading about the
latest releaase.  It is better that we send users to istanbul.js.org for
documentation.

Issue istanbuljs#1200

package.json

@@ -100,6 +100,7 @@
   "engines": {
     "node": ">=8"
   },
+  "homepage": "https://istanbul.js.org/",
   "repository": {
     "type": "git",
     "url": "git@github.com:istanbuljs/nyc.git"