Create CONTRIBUTING.md

Author: RobinBuschmann

CONTRIBUTING.md

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+To contribute you can:
+- Open issues and participate in discussion of other issues.
+- Fork the project to open up PR's.
+- Update the [types of Sequelize](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/sequelize).
+- Anything else constructively helpful.
+
+In order to open a pull request please:
+- Create a new branch.
+- Run tests locally (`npm install && npm run build && npm run cover`) and ensure your commits don't break the tests.
+- Document your work well with commit messages, a good PR description, comments in code when necessary, etc.
+
+In order to update the types for sequelize please go to [the Definitely Typed repo](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/sequelize), it would also be a good
+idea to open a PR into [sequelize](https://github.com/sequelize/sequelize) so that Sequelize can maintain its own types, but that
+might be harder than getting updated types into microsoft's repo. The Typescript team is slowly trying to encourage
+npm package maintainers to maintain their own typings, but Microsoft still has dedicated and good people maintaining the DT repo, 
+accepting PR's and keeping quality high.
+
+**Keep in mind `sequelize-typescript` does not provide typings for `sequelize`** - these are seperate things.
+A lot of the types in `sequelize-typescript` augment, refer to, or extends what sequelize already has.