chore: Create CONTRIBUTING.md with contribution guidelines (#331)

Added guidelines for contributing to the project, including testing,
submitting changes, and coding conventions.

Author: gtema

CONTRIBUTING.md

@@ -0,0 +1,45 @@
+# How to contribute
+
+We are really glad you're reading this, because we need volunteer developers
+to help this project come to fruition.
+
+Here are some important resources:
+
+  * [OpenStack contribution guide](https://docs.openstack.org/contributors/index.html)
+  * Bugs? [GitHub issues](https://github.com/openstack-experimental/keystone/issues)
+  * IRC: chat.oftc.net channel [#openstack-keystone](https://docs.openstack.org/contributors/common/irc.html).
+    We're spread across the globe, hopefully close to your TZ.
+
+## Testing
+
+We try to implement unit and functional testing for every piece of the
+functionality. In the federation area we try to ensure that we have a
+real functional tests for every available provider.
+
+## Submitting changes
+
+Please send a [GitHub Pull Request](https://github.com/openstack-experimental/keystone/pull/new/main)
+with a clear list of what you've done (read more about
+[pull requests](http://help.github.com/pull-requests/)).  Please follow our
+coding conventions (below) and make sure all of your commits are atomic
+(one feature per commit).
+
+Since our target for the project is to become official OpenStack project we
+would require Signed-off in the commit message sometime soon.
+
+Always write a clear log message for your commits. One-line messages are fine
+for small changes, but bigger changes should look like this:
+
+    $ git commit -s -m "A brief summary of the commit
+    >
+    > A paragraph describing what changed and its impact."
+
+## Coding conventions
+
+Start reading our code and hopefully it is reasonable well documented. If not
+than please help us
+
+* Use "Result<." and "?" to properly propagate errors
+* Pass by reference when receiver is not supposed to take ownership
+* "thiserror" is used to define internal errors. Ensure that enough context
+  information is available in the error.

doc/src/SUMMARY.md

@@ -16,7 +16,7 @@
     * [Passkey Auth](adr/0005-auth-passkey.md)
     * [Federation IDP](adr/0006-federation-idp.md)
     * [Federation Mapping](adr/0007-federation-mapping.md)
-    * [Workload Federation](adr/0008-workload-federation.md) 
+    * [Workload Federation](adr/0008-workload-federation.md)
 * [Policy enforcement](./policy.md)
 * [Fernet token]()
   * [Token payloads]()