Add contribution guideline (#210)

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# How to contribute
+
+Thank you for your contribution to grpc-gateway.
+Here's the recommended process of contribution.
+
+1. `go get github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway`
+2. `cd $GOPATH/src/github.com/grpc-ecosystem/grpc-gateway`
+3. hack, hack, hack...
+4. Make sure that your change follows best practices in Go
+   * [Effective Go](https://golang.org/doc/effective_go.html)
+   * [Go Code Review Comments](https://golang.org/wiki/CodeReviewComments)
+5. Make sure that `make test` passes.
+6. Sign [a Contributor License Agreement](https://cla.developers.google.com/clas)
+7. Open a pull request in Github
+
+When you work on a larger contribution, it is also recommended that you get in touch
+with us through the issue tracker.
+
+### Code reviews
+All submissions, including submissions by project members, require review.

README.md

@@ -219,6 +219,9 @@ But patch is welcome.
 * Encoding request/response body in XML
 * True bi-directional streaming. (Probably impossible?)
 
+# Contribution
+See [CONTRIBUTING.md](http://github.com/grpc-ecosystem/grpc-gateway/blob/master/CONTRIBUTING.md).
+
 # License
 grpc-gateway is licensed under the BSD 3-Clause License.
 See [LICENSE.txt](https://github.com/grpc-ecosystem/grpc-gateway/blob/master/LICENSE.txt) for more details.