docs: add the default CLA

CONTRIBUTING.markdown

@@ -0,0 +1,7 @@
+# Copyright Assignment
+
+Thank you for your contribution. Here is some important legal stuff.
+
+By submitting a pull request for this project, unless explicitly stated otherwise, you agree to assign your copyright of the contribution to **The RedPRL Development Team** when it is accepted (merged with or without minor changes). You assert that you have full power to assign the copyright, and that any copyright owned by or shared with a third party has been clearly marked with appropriate copyright notices. If you are employed, please check with your employer about the ownership of your contribution.
+
+This would allow us to, for example, change the license of the codebase to Apache 2.0 or transfer the ownership of the project to someone else *without your further consent*. We demand this assignment so that we do not have to ask *everyone* who has ever contributed for these activities. This requires trust, and if you feel uncomfortable about this assignment, please make an explicit note.