Update README.md

Was reading the README to consider using the project for JSON generation and noticed some places where it could be cleaned up a bit. Thanks for your work on this project.

* Updates Markdown format to use GitHub flavor for code.
* Updates content with some organizational changes, grammar, etc.
* Adds jbuilder in related projects.
* Updates indentation in pretty JSON to 4 spaces.
Merge pull request #15 from elskwid/patch-1

Update README.md