Add contribution guidelines

CONTRIBUTING.md

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+Thanks for helping us build Recurly's Go library! This page contains requirements and guidelines for contributions.
+
+#### Requirements
+* New objects must include type assertions in the service and mock. 
+* New services need to be added to `services.go` and in the `mock` package. 
+* This package uses `cmp` instead of `reflect` for tests. All submissions must include tests. 
+* Please follow the Go Standard Library style (for example, line comments should end with a period but inline comments should not).  
+
+#### Problems or questions?
+The best place to ask dev related questions is in `Issues` for this repo.