Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Gradio UI
+You can start by forking or cloning the repo (https://github.com/gradio-app/gradio-UI.git) and creating your own branch to work from. All PRs must pass the continuous integration 
+tests and receive approval from a member of the Gradio UI development team before they will be merged.
+
+### Docstrings
+
+We expect all PRs to add or update API documentation for any affected pieces of code.
+We use [NumPy style docstrings](https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_numpy.html), and enforce style compliance with pydocstyle as indicated above.
+Docstrings can be cumbersome to write, so we encourage people to use tooling to speed up the process.
+For VSCode, we like [autoDocstring](https://marketplace.visualstudio.com/items?itemName=njpwerner.autodocstring).
+Just install the extension and add the following configuration to the `settings.json` example above.
+Note that we use PEP 484 type hints, so parameter types should be removed from the docstring (although note that return types should still be included).
+
+```json
+{
+    "autoDocstring.docstringFormat": "numpy",
+    "autoDocstring.guessTypes": false
+}
+```
+
+
+### Continuous Integration and Testing
+All PRs must pass the continuous integration tests before merging. To test locally, you can run python3 -m unittest.
+
+
+### Submitting PRs
+
+All PRs should be against `master`. Direct commits to master are blocked, and PRs require an approving review
+to merge into master. By convention, the Gradio UI maintainers will review PRs when:
+  * An initial review has been requested
+  * A maintainer is tagged in the PR comments and asked to complete a review
+
+We ask that you make sure initial CI checks are passing before requesting a review.
+One of the Gradio UI maintainers will merge the PR when all the checks are passing.