Add contribution guidance

Add CONTRIBUTING.md that points to the same document in chef-cookbooks
Add mention of the contribution document in the top of the README.md
Merge pull request #331 from claco/contributing

Merged automatically by jenkins after successful build test
ha clustering set from rabbitmq settings
Merge pull request #332 from rackerjoe/master

Merged automatically by jenkins after successful build test
DE264: Change nova.scheduler.filters.standard_filters to nova.scheduler.filters.all_filters