Document JoinTable migration generator
Update documentation for yml usage

Update documentation for yml usage against https://github.com/rails/rails/issues/8612
Update guides/source/layouts_and_rendering.md

Removed first slash. The subdirectory will nto be found with slash befor it.

fixed inconsistent `require 'test_helper'` in testing guide
Update guides/source/rails_on_rack.md

Rack::Lock actually sets rack.multithread to `false`, not `true`.

Presumably because once inside the mutex, the app can function as if it were single threaded.
Add documentation for the TimeWithZone#dst? method.

[ci skip]
Add documentation for TimeWithZone methods
Improve English

* Fix one error
* Improve wording
* Improve punctuation
Merge pull request #123 from m-o/master

Fixed typo one line in layouts and rendering: stylesheet_link_tag for file in subdirectory.
copy edits [ci skip]
Merge branch 'master' of github.com:lifo/docrails