Add short contributing guide (#907)

Closes #241

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contributing
+
+We welcome contributions to the project! To propose a change, please fork the repository, make your changes, then submit
+a pull request. If the change is significant or potentially controversial, please open an issue first to discuss it.
+Zoekt does not require a CLA to contribute.
+
+Before opening a pull request, make sure that you have run the tests locally:
+```sh
+go test ./...
+```
+
+It's also good to run a local smoke test for the relevant component. For example, if you've made changes in
+`zoekt-git-index`, you can try indexing a repository locally:
+```sh
+go run ./cmd/zoekt-git-index /path/to/repo
+```
+