add comment about why 600 is used

Author: Jonathon Belotti

defs.bzl

@@ -44,6 +44,7 @@ pip_repository = repository_rule(
         "requirements": attr.label(allow_single_file=True, mandatory=True,),
         "wheel_env": attr.string_dict(),
         "python_interpreter": attr.string(default="python3"),
+        # 600 is documented as default here: https://docs.bazel.build/versions/master/skylark/lib/repository_ctx.html#execute
         "timeout": attr.int(default = 600),
         "_script": attr.label(
             executable=True, default=Label("//src:__main__.py"), cfg="host",