GitHub supports several lightweight markup languages for documentation; the most popular ones (generally, not just at GitHub) are Markdown and reStructuredText. Markdown is sometimes considered easier to use, and is often preferred when the purpose is simply to generate HTML. On the other hand, reStructuredText is more extensible and powerful, with native support (not just embedded HTML) for tables, as well as things like automatic generation of tables of contents.
It's pretty easy to do polymorphic associations in Rails: A Picture can belong to either a BlogPost or an Article. But what if you need the relationship the other way around? A Picture, a Text and a Video can belong to an Article, and that article can find all media by calling @article.media
This example shows how to create an ArticleElement join model that handles the polymorphic relationship. To add fields that are common to all polymorphic models, add fields to the join model.
Sublime Text 2 ships with a CLI called subl (why not "sublime", go figure). This utility is hidden in the following folder (assuming you installed Sublime in /Applications like normal folk. If this following line opens Sublime Text for you, then bingo, you're ready.
open /Applications/Sublime\ Text\ 2.app/Contents/SharedSupport/bin/subl
You can find more (official) details about subl here: http://www.sublimetext.com/docs/2/osx_command_line.html