all: add CONTRIBUTING.md

Change-Id: I3fcf268e35159e89beacf1ec63dfc256477a6382
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220678
Reviewed-by: Damien Neil <[email protected]>

Author: dsnet

CONTRIBUTING.md

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+# Contributing to Go Protocol Buffers
+
+Go protocol buffers is an open source project and accepts contributions.
+
+This project is the first major revision of Go protobufs,
+while the next major revision of this project is located at
+[protocolbuffers/protobuf-go](https://github.com/protocolbuffers/protobuf-go).
+Most new development effort is focused on the latter project,
+and changes to this project is primarily reserved for bug fixes.
+
+
+## Contributor License Agreement
+
+Contributions to this project must be accompanied by a Contributor License
+Agreement. You (or your employer) retain the copyright to your contribution,
+this simply gives us permission to use and redistribute your contributions as
+part of the project. Head over to <https://cla.developers.google.com/> to see
+your current agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different project), you probably don't need to do it
+again.
+
+
+## Code reviews
+
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.