Remove development doc in favor of wiki

The docs folder should contain info about running Marathon in production
environments. Content related to developing Marathon itself should be
maintained in the GitHub Wiki.

This doc was moved to:
https://github.com/mesosphere/marathon/wiki/Developing-Marathon-in-Vagrant
Add README section about developing Marathon

It points to the GitHub project wiki.
Expand documentation site dependencies list

* TODO: Add OS X instructions for installing dependencies (probably
  Homebrew).
Merge pull request #569 from mesosphere/dev-docs-in-wiki

Remove development doc in favor of wiki