Add CONTRIBUTING.md (#585)

## Changes
<!-- Summary of your changes that are easy to understand -->

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
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- [ ] `make test` run locally
- [ ] `make fmt` applied
- [ ] relevant integration tests applied

Author: mgyucht

CONTRIBUTING.md

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+We happily welcome contributions to the Databricks SDK for Python. We use [GitHub Issues](github.com/databricks/databricks-sdk-py/issues) to track community reported issues and [GitHub Pull Requests](https://github.com/databricks/databricks-sdk-py/pulls) for accepting changes.
+Contributions are licensed on a license-in/license-out basis.
+
+# Contributing Guide
+
+## Communication
+Before starting work on a major feature, please open a GitHub issue. We will make sure no one else is already working on it and that it is aligned with the goals of the project.
+A "major feature" is defined as any change that is > 100 LOC altered (not including tests), or changes any user-facing behavior.
+We will use the GitHub issue to discuss the feature and come to agreement.
+This is to prevent your time being wasted, as well as ours.
+The GitHub review process for major features is also important so that organizations with commit access can come to agreement on design.
+If it is appropriate to write a design document, the document must be hosted either in the GitHub tracking issue, or linked to from the issue and hosted in a world-readable location.
+Small patches and bug fixes don't need prior communication.
+
+## Coding Style
+Code style is enforced by a formatter check in your pull request. We use [yapf](https://github.com/google/yapf) to format our code. Run `make fmt` to ensure your code is properly formatted prior to raising a pull request.
+
+## Signed Commits
+This repo requires all contributors to sign their commits. To configure this, you can follow [Github's documentation](https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits) to create a GPG key, upload it to your Github account, and configure your git client to sign commits.
+
+## Sign your work
+The sign-off is a simple line at the end of the explanation for the patch. Your signature certifies that you wrote the patch or otherwise have the right to pass it on as an open-source patch. The rules are pretty simple: if you can certify the below (from developercertificate.org):
+
+```
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+Then you just add a line to every git commit message:
+
+```
+Signed-off-by: Joe Smith <[email protected]>
+```
+
+If you set your `user.name` and `user.email` git configs, you can sign your commit automatically with git commit -s.
+You must use your real name (sorry, no pseudonyms or anonymous contributions).