Add missing file

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# How to contribute
+
+We'd love to accept your patches and contributions to this project.
+There are just a few small guidelines you need to follow.
+
+- Unless you explicitly state otherwise, any contribution intentionally
+  submitted for inclusion in work by you shall be dual licensed under MIT
+  license or Apache-2.0 license without any additional terms or conditions.
+- All submissions, including submissions by project members, require review.
+  We use GitHub pull requests for this purpose.
+  Consult GitHub Help for more information on using pull requests.
+
+We require all contributors to follow the [daac-tools Code of
+Conduct](https://github.com/daac-tools/guidelines/blob/main/conduct.md).