document how to create custom rails environments
Merge pull request #9019 from senny/doc_custom_rails_env

document how to create custom rails environments
copy-edits 7e6e4f2 [ci skip]

Capitalization in the title of the section, enumerates the environments
in the usual order, adds a paragraph, and tries to use less "you"s.
Fix asset_path in mounted engine

Historically serving assets from a mountable engine could be achieved by
running ActionDispatch::Static as a part of engine middleware stack or
to copy assets prefixed with an engine name. After introduction of
assets pipeline this is not needed as all of the assets are served or
compiled into main application's assets.

This commit removes the obsolete line making asset_path always generate
paths relative to the root or config.relative_url_root if it's set.

(closes #8119)
Merge pull request #9021 from drogus/fix-asset-path

Fix asset_path in mounted engine
Update actionpack's CHANGELOG for 445f14e