Added initial draft of contributor guidelines.
Cleaned up examples, added Getting Started headline
Added note about regression tests.
Replace random service ports in docker mappings.

- Fixes #714
Merge pull request #715 from mesosphere/contributor-guidelines

Added initial draft of contributor guidelines.
GroupManager: edit for clarity, rm unnecessary ids
Merge pull request #735 from mesosphere/714-bridge-random-service-ports

Replace random service ports in docker mappings.
Added ability to handle CentOS/RHEL systems.
Turn on HAProxy service for CentOS/RHEL.
Add exit for when an unsupported OS is found.
Fixed my indents. Replaced haproxy.cfg with a variable.
Update download links.
Merge pull request #736 from potto007/master

Add support for CentOS/RHEL to haproxy-marathon-bridge
validate protocol in port mapping
Merge pull request #750 from mesosphere/wip-portmapping-validation-drexin

validate protocol in port mapping