add CONTRIBUTING.md

Author: lucacasonato

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+To build the spec locally first install bikeshed:
+
+```sh
+pip3 install bikeshed && bikeshed update
+```
+
+```sh
+bikeshed spec # build once
+
+# or
+
+bikeshed watch # rebuild on changes
+```