Renaming README to use Markdown

Also used the long copy form I wanted to test out.
Updated RDoc tags for markdown
Update README.md
Fixed grammar and typos
fix prism.js's link in README
Merge pull request #16 from jipiboily/patch-1

fix prism.js's link in README
Removed obsolete images
Merge branch 'master' of github.com:pothibo/ecrire
A more welcoming message for new user :)
Added a missing gems that would make it impossible to start the server

therubyracer is a gem based on V8 which is needed to compile assets.
Sprocket fails to start if it can't find an engine to process assets.
Added link to the readme to access the Howto
Fixed typo in database settings.

Not sure why this wasn't found earlier but configuration try to find a
username while settings.yml specify a user key which result in a nil
username.

postgres defaults to the current user when none are specified which was
giving confusing errors.
Merge branch 'master' of github.com:pothibo/ecrire