Update 'gotchas' documentation for NoErrorsPlugin

[edmorley]

The readme gotchas section currently mentions that using NoErrorsPlugin results in no bundle output if eslint errors occurred in eslint-loader.

However:

• that plugin was replaced by NoEmitOnErrorsPlugin
• NoEmitOnErrorsPlugin is now automatically enabled in webpack 4, when mode is either unset, or set to production:
  • https://github.com/webpack/webpack/blob/e1df721fd77634b5a22b4349c7d7ea15f0a2188e/lib/WebpackOptionsDefaulter.js#L267-L269
  • https://github.com/webpack/webpack/blob/3b6d1495c3858d59cfdb46b2343c07f2e4c1316d/lib/WebpackOptionsApply.js#L364-L366
• it appears that "So even ESLint warnings will fail the build. No matter what error settings are used for eslint-loader" reference is no longer true (since emitWarning: true worked, even with --mode production).

[dagadbm]

so how can I make eslint not break then?