ADD a CONTRIBUTING.md that is shown to users when they open a PR or create an issue
Some additions
Merge pull request #201 from Shopify/contributing

Add a CONTRIBUTING.md
Fix bug #179

Also reported at https://github.com/Shopify/sarama/pull/199#issuecomment-63636565

Go's closure semantics are really annoying - simply stop spawning goroutines
with the wrong arguments.

Add a test (heavily based on https://gist.github.com/ORBAT/d0adcd790dff34b37b04)
to ensure this behaviour doesn't regress.

Huge thanks to Tom Eklöf for getting me all the logs etc. needed to track this
down.
Merge pull request #202 from Shopify/fix-179

Fix bug #179
Replace successes channel with individual promises

Makes tracking success of individual messages much simpler and friendly to
order-dependent things. Also provides a more efficient solution to the first
half of #187 (it lets us get rid of the mutex in SimpleProducer).