Add contributing guide (#6)

* Add contributing guide

Co-authored-by: Karl Tarbe <[email protected]>

Author: fboemer

CONTRIBUTING.md

@@ -0,0 +1,7 @@
+## Contributing to Swift Homomorphic Encryption
+
+Welcome to the Swift Homomorphic Encryption community! Thanks for your interest in contributing. We would love contributions in the form of feedback via [GitHub Issues](https://github.com/apple/swift-homomorphic-encryption/issues), pull requests with new features, bug fixes, documentation (including fixing typos!), and your ideas. If it's a big change, please start with an Issue.
+
+### Pull Requests:
+Before making a commit for a pull request, please run `pre-commit install`.
+Then on each commit some basic formatting checks will be run.

README.md

@@ -167,7 +167,7 @@ Two ways to run the benchmarks are:
   * See the [Benchmark](https://github.com/ordo-one/package-benchmark) documentation for more information on running benchmarks.
 
 ### Contributing
-If you would like to make a pull request to Swift Homomorphic Encryption, please run `pre-commit install`. Then each commit will run some basic formatting checks.
+If you are interested in making a contribution to Swift Homomorphic Encryption, see our [contributing guide](CONTRIBUTING.md).
 
 # Documentation
 Swift Homomorphic Encryption uses DocC for documentation.