initial contrib doc

CONTRIBUTING.md

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+# How to Contribute
+
+## GitHub Workflow
+
+1. Fork the druid-io/druid repository into your GitHub account
+
+https://github.com/druid-io/druid
+
+2. Clone your fork of the GitHub repository
+
+```sh
+git clone git@github.com:<ghuser>/druid.git
+```
+
+Add a remote to keep up with upstream changes
+
+```
+git remote add upstream https://github.com/druid-io/druid.git
+```
+
+If you already have a copy, fetch upstream changes
+
+```
+git fetch upstream
+```
+
+3. Create a feature branch to work in
+
+```
+git checkout -b feature-xxx remotes/upstream/master
+```
+
+4. Work in your feature branch
+
+```
+git commit -a
+```
+
+5. Periodically rebase your changes
+
+```
+git pull --rebase
+```
+
+6. When done, "squash" your commits
+
+```
+git rebase -i upstream/master
+```
+
+Prefix commits using `s` (squash) or `f` (fixup) to merge extraneous commits.
+
+7. Submit a pull-request
+
+```
+git push origin feature-xxx
+```
+
+Go to your Druid fork main page
+
+https://github.com/<ghuser>/druid
+
+If you recently pushed your changes GitHub will automatically pop up a
+`Compare & pull request` button for any branches you recently pushed to. If you
+click that button it will automatically offer you to submit your pull-request
+to the druid-io/druid repository.
+
+- Give your pull-request a meaningful title.
+- In the description, explain your changes and the problem they are solving.
+
+8. Addressing code review comments
+
+Repeat steps 4. through 6. to address any code review comments and
+rebase your changes if necessary.
+
+Push your updated changes to update the pull request
+
+```
+git push origin [--force] feature-xxx
+```
+
+`--force` may be necessary to overwrite your existing pull request in case your
+commit history was changed when performing the rebase.
+
+Note: Be careful when using `--force` since you may lose data if you are not careful.
+
+```
+git push origin --force feature-xxx
+```
+
+
+# FAQ
+
+### Help! I merged changes from upstream and cannot figure out how to resolve conflits when rebasing!
+
+Never fear, if you occasionally merged upgstream/master, here is another way to squash your changes into a single commit
+
+Rename your existing branch first
+
+```
+git branch -m feature-xxx-unclean
+```
+
+Checkout a new branch with the original name `feature-xxx` from upstream. This branch will supercede our old one.
+
+```
+git checkout -b feature-xxx upstream/master
+```
+
+Then merge your changes in your original feature branch `feature-xxx-unclean` and create a single commit.
+
+```
+git merge --squash feature-xxx-unclean
+git commit
+```
+
+You can now submit this new branch and create or replace your existing pull request
+
+```
+git push origin [--force] feature-xxx:feature-xxx
+```