ci: add CLA

Author: rubenfiszel

.github/workflows/sign-cla.yml

@@ -19,8 +19,7 @@ jobs:
         with:
           path-to-signatures: "signatures/cla.json"
           path-to-document: "https://github.com/windmill-labs/windmill/blob/master/CLA.md"
-          # branch should not be protected
-          branch: "main"
+          branch: "signatures"
           allowlist: rubenfiszel,bot*
 
           #below are the optional inputs - If the optional inputs are not given, then default values will be taken