Warning on doc for assets under lib|vendor not precompiling
Soften wording about knowing other devs.

Many people don't, so point them to a place where they can meet
some.
Flesh out feedback portion of contributing guide.

I don't know of anyone that hangs out in #rails-contrib, and we should
also let people know that feedback can take some time.
Add note about features vs. bugs.

Sometimes people ask what makes a feature and what makes a bug, so
let's clarify that so we can show it to people later.
Merge pull request #11986 from fedesoria/patch-3

Warning on doc for assets under lib|vendor not precompiling
Remove whiny_nils config from the rails guides

This option was removed from master.

[ci skip]
Merge pull request #12032 from carolinemota/fixdoc

Remove whiny_nils config from the rails guides [ci skip]