Removes an unused type parameter from Nondeterminism aggregate calls.

Allow monoid instance for Nondeterminism aggregate calls to be implicit.
improve deprecation message
rename DeveloperGuide.md to CONTRIBUTING.md

GitHub gives special treatment to this file
https://github.com/blog/1184-contributing-guidelines
Merge pull request #612 from nuttycom/bug/remove_unused_nondeterminism_type_parameter

Removes an unused type parameter from Nondeterminism aggregate calls.