Adding a CONTRIBUTING document

Adding a CONTRIBUTING document to take advantages of this:
https://github.com/blog/1184-contributing-guidelines
Issue 1712 - Incorrect error notification when creating user

No need to validate rolename when role  == self_role
965724 - install foreman-proxy-installer as a part of katello-foreman-all
967467 - search for env by label when not found by name only for rhsm

Otherwise, katello-cli behaves strangely when addressing environment
by name, and it uses the label instead.
Merge pull request #2373 from mptap/rolename_validator

Issue 1712 - Incorrect error notification when creating user
Merge pull request #2333 from daviddavis/temp_1368975142

Adding a CONTRIBUTING document
Merge pull request #2407 from iNecas/bz/967467

967467 - search for env by label when not found by name only for rhsm
Merge pull request #2406 from iNecas/bz/965724

965724 - install foreman-proxy-installer as a part of katello-foreman-all
We need new ldap_fluff for Fedora
Merge pull request #2408 from ares/ldap_fluff_update

We need new ldap_fluff for Fedora
859106 - UI fix - Missing Save/Cancel buttons in system change environment
Merge pull request #2378 from jhadvig/missing_env_buttons_fix

859106 - UI fix - Missing Save/Cancel buttons in system change environme...