Giving the ability to open a Markup (.md) file in SourceTree through an overlay popup preview similar to how JIRA & Confluence render selected image attachments would be hugely beneficial for the end-user who may be disconnected from Internet access & wishes to easily view their README.md file for example offline.

Future interactions could even include a WYSIWYG type editor introduced into the diff area of SourceTree.

A great example of this is http://github-markup.dfilimonov.com/

Moreover,
By having such a feature supported, Atlassian / SourceTree would have one up on the Git GUI competition