Warning on doc for assets under lib|vendor not precompiling

[fedesoria]

There is no reference to having assets under lib/assets or vendor/assets that they will not be precompiled.
Took me a while to figure this out since it's only documented on the upgrade notes.

[fxn]

Proposed edit:

WARNING: If you are upgrading from Rails 3, please take into account that assets
under `lib/assets` or `vendor/assets` are available for inclusion via the
application manifests but no longer part of the precompile array. See
[Precompiling Assets](#precompiling-assets) for guidance.

[robin850]

👍

[fedesoria]

All done.
Thanks for the help, my first rails commit.

[fxn]

Awesome, thanks!

[fxn]

@fedesoria welcome! http://contributors.rubyonrails.org/contributors/federico-soria/commits