add a CONTRIBUTING file

Author: krichprollsch

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# Contributing
+
+Lightpanda accepts pull requests through GitHub.
+
+You have to sign our [CLA](CLA.md) during your first pull request process
+otherwise we're not able to accept your contributions.
+
+The process signature uses the [CLA assistant
+lite](https://github.com/marketplace/actions/cla-assistant-lite). You can see
+an example of the process in [#303](https://github.com/lightpanda-io/browser/pull/303).