Ignore plugin is missing from docs

[glenjamin]

Webpack v1 docs include a section about IgnorePlugin - although it is a tad cryptic.

There's no equivalent entry in the webpack 2 docs.

It would be good to add this and provide an explanation of what the second argument does that's easier to understand if you aren't familiar with webpack internal terms (or link to an explanation of those terms?).

[welll]

@skipjack does someone need to add this link at index, right?

Index Plugins Page:
https://webpack.js.org/plugins/

Source:
https://github.com/webpack/webpack.js.org/blob/master/content/plugins/index.md

If yes, just create a new ISSUE and I could do that...

[skipjack]

@welll yea the index is missing a few links/descriptions I think. Feel free to open a a pr and request me as the reviewer. No need to open a new issue...

[welll]

@skipjack I couldn't add you as reviewer

#1232