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.
969216 - retrieve org by label when org identified in pool details

When katello gets a pool from candlepin, the 'owner key' is
equivalent to the organization label.  This change is to
properly retrieve org by label vs name.
915900 - Systems - Events History - Clear search does not work

Elastic search query was wrong and was not able to load system_event.js due to which the right results were not pulled.So we updated the type of task_owner_type in the search query which fixed the issue.
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...
Merge pull request #2390 from pgodhani/bugfix

915900 - Systems - Events History - Clear search does not work
default_info - converting apply to sync

- db migration adding a new 'default' column for custom_info
- updating the schema
- re-wording from 'apply' to 'sync'
- fixing spec tests

default_info "apply" takes the default_info keys that are specified on
an org, and syncs them with all the systems (or distributors) under that
org. this includes adding custom_info but also removing custom_info
(that may have values attached to it).
969001 - requiring org when calling Product.find_by_cp_id

Products do not have a unique cp_id, as redhat products are
singular within candlepin
966027 - hiding ability to refresh a manifest if its not supported
726401: Systems page - needs better error behavior when tomcat6 is not running

ECONNREFUSED error shows up when tomcat6 is not running.So we handled that exception by throwing user-friendly error.In order for it to work we had to handle exception in http_resource.rb and defined a custom exception - ConnectionRefusedException in errors.rb.
Merge pull request #2404 from komidore64/default-info-sync

default_info - converting apply to sync