Docs: Provide install commands for Yarn

[nbaksalyar]

npx doesn't work properly with plugins installed with Yarn.

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

• Documentation update

What changes did you make? (Give an overview)

The installation guide might be slightly confusing for new users - e.g.: I use TypeScript in my project, I install eslint with yarn as the guide says, and then run it with npx eslint --init. It then fails with an error because the TypeScript plugin has been installed with yarn and npx doesn't see it:

Error: Failed to load plugin '@typescript-eslint' declared in '.eslintrc.js': Cannot find module 'eslint'

This can be fixed by using yarn run <cmd> instead of npx <cmd>. This PR changes documentation to reflect this.

[jsf-clabot]

All committers have signed the CLA.

[btmills]

Thanks for contributing, @nbaksalyar!

npx doesn't work properly with plugins installed with Yarn.

I didn't know that, but that's annoying. Is that because of the installation changes in Yarn v2? (I tried it with Yarn v1 and didn't have any issues.) Either way, LGTM because I'm fine with having both npm/npx and yarn commands for consistency.

[nbaksalyar]

Is that because of the installation changes in Yarn v2?

I think so! I use the berry (v2.2) version for Yarn.

Thanks for reviewing :)

[kaicataldo]

Thanks for contributing!