Add contributors docs

.github/CONTRIBUTING.md

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+How to contribute
+=================
+
+Thanks for your interest in contributing!
+
+## Reporting Bugs
+
+Report bugs at https://github.com/Cornices/cornice/issues/new
+
+If you are reporting a bug, please include:
+
+ - Any details about your local setup that might be helpful in troubleshooting.
+ - Detailed steps to reproduce the bug or even a PR with a failing tests if you can.
+
+
+## Ready to contribute?
+
+### Getting Started
+
+ -  Fork the repo on GitHub and clone locally:
+
+```bash
+git clone [email protected]:Cornices/cornice.git
+git remote add {your_name} [email protected]:{your_name}/cornice.git
+```
+
+## Testing
+
+ -  `make test` to run all the tests
+
+## Submitting Changes
+
+```bash
+git checkout main
+git pull origin main
+git checkout -b issue_number-bug-title
+git commit # Your changes
+git push -u {your_name} issue_number-bug-title
+```
+
+Then you can create a Pull-Request.
+Please create your pull-request as soon as you have at least one commit even if it has only failing tests. This will allow us to help and give guidance.
+
+You will be able to update your pull-request by pushing commits to your branch.
+
+
+## Releasing
+
+1. Create a release on Github on https://github.com/Cornices/cornice/releases/new
+2. Create a new tag `X.Y.Z` (*This tag will be created from the target when you publish this release.*)
+3. Generate release notes
+4. Publish release