Add Markdown suffix to `README`

For better markup capabilities.
Enhance `README` markup

Links, lists, headers, etc.
Update license information

We mean to use LGPLv3, not GPLv3. The former allows a shared library to
be linked from non-free software; the latter does not.

Also updated copyright declarations (first year of release through
current release year).
Remove contact email

Users should open GitHub issues to report bugs.
Add guidelines for contributors
Add support policy

To set expectations.
Small wording/formatting changes to README
Return email to README

Per @ozgune's request.
Merge pull request #15 from citusdata/feature/update_docs

Update project documentation