Updated contributed.md.

The PR adds information and best practises pertaining to contributing to the `Idris` prelude and base.

This informaton has been culled from information on the Idris Wiki and past pull requests, and only supports existing judgements made.

See:

+ https://github.com/idris-lang/Idris-dev/commit/ba6f0ef10086f1d316dedc11164330fecb979d01
+ https://github.com/idris-lang/Idris-dev/wiki/Idris-Developers-Meeting,-April-May-2014
Merge pull request #1645 from jfdm/update-contrib

Updated contributed.md.