Add a contributing guide

CONTRIBUTING.md

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+## How to contribute code
+
+Follow these steps to submit your code contribution.
+
+### Step 1. Open an issue
+
+Before making any changes, we recommend opening an issue (if one doesn't already
+exist) and discussing your proposed changes. This way, we can give you feedback
+and validate the proposed changes.
+
+If the changes are minor (simple bug fix or documentation fix), then feel free
+to open a PR without discussion.
+
+### Step 2. Make code changes
+
+To make code changes, you need to fork the repository. You will need to setup a
+development environment and run the unit tests. This is covered in section
+"Setup environment".
+
+### Step 3. Create a pull request
+
+Once the change is ready, open a pull request from your branch in your fork to
+the master branch in [keras-team/keras](https://github.com/keras-team/keras).
+
+### Step 4. Sign the Contributor License Agreement
+
+After creating the pull request, the `google-cla` bot will comment on your pull
+request with instructions on signing the Contributor License Agreement (CLA) if
+you haven't done so. Please follow the instructions to sign the CLA. A `cla:yes`
+tag is then added to the pull request.
+
+![Tag added](https://i.imgur.com/LHEdIfL.png)
+
+
+### Step 5. Code review
+
+CI tests will automatically be run directly on your pull request.  Their
+status will be reported back via GitHub actions.
+
+There may be
+several rounds of comments and code changes before the pull request gets
+approved by the reviewer.
+
+![Approval from reviewer](https://i.imgur.com/zgRziTt.png)
+
+### Step 6. Merging
+
+Once the pull request is approved, a team member will take care of merging.
+
+## Setup environment
+
+Setting up your KerasCV development environment is quite easy.  You simply
+need to run the following commands:
+
+```shell
+git clone https://github.com/YOUR_GITHUB_USERNAME/keras-cv.git
+cd keras-cv
+pip install ".[tests]"
+python setup.py develop
+```
+
+Following these commands you should be able to run the tests using `pytest keras_cv`.
+Please report any issues running tests following these steps.
+
+## Run tests
+
+KerasCV is tested using [PyTest](https://docs.pytest.org/en/6.2.x/).
+
+### Run a test file
+
+To run a test file, run `pytest path/to/file` from the root directory of keras\_cv.
+
+### Run a single test case
+
+To run a single test, you can use `-k=<your_regex>`
+to use regular expression to match the test you want to run. For example, you
+can use the following command to run all the tests in `cut_mix_test.py`,
+whose names contain `label`,
+
+```
+pytest keras_cv/layers/preprocessing/cut_mix_test.py -k="label"
+```
+
+### Run all tests
+
+Running all of the tests in KerasCV is as easy as:
+```
+pytest keras_cv/
+```
+
+### Formatting the Code
+In order to format the code you can use the `shell/format.sh` script.
+If this does not resolve the issue, try updating `isort` and `black`
+via `pip install --upgrade black` and `pip install --upgrade isort`.