Add contribution guidelines.

CONTRIBUTING.md

+# Contribute to DGraph
+
+**Pull requests are welcome.**
+
+## Style Guide
+We're following [Go Code Review](https://github.com/golang/go/wiki/CodeReviewComments).
+Use `gofmt` for formatting your code. Avoid unnecessary vertical spaces.
+Wrap your code and comments to 100 characters, unless doing so makes the code less legible.
+
+## Code Review Workflow
+- All contributors need to sign the [Contributor License Agreement](https://cla-assistant.io/dgraph-io/dgraph).
+- Pick an issue from [pending issues](https://github.com/dgraph-io/dgraph/issues).
+- If the issue isn't currently present in the list, file a new issue,
+or bring it up on [Google Group](https://groups.google.com/forum/#!forum/dgraph) or [Gitter Chat](https://gitter.im/dgraph-io/dgraph),
+so someone can file an issue on your behalf.
+- Indicate that you're working on the issue by assigning the issue to yourself.
+- Create a new branch, write your code, and commit your changes locally.
+- Make sure you run `go test ./...` from the root.
+- [Create a pull request](https://help.github.com/articles/creating-a-pull-request/)
+- DGraph uses [Reviewable](https://reviewable.io/) for code reviews, and follows a rigorous code review process.
+- Address the comments, and repeat the cycle, until you get an LGTM by someone qualified.
+- Once you have an LGTM, go ahead and merge.
+Most new contributors aren't allowed to merge themselves, in that case, we'll do it for you.