document permission_mask bit meanings

README.md

@@ -89,6 +89,10 @@ The following 4 fields are all optional and default to `false` or `0` if omitted
 - `default_enabled` (`boolean`) should the component be enabled by default?
 - `first_party_protections` (`boolean`) should first-party heuristics be applied to rules from the list?
 - `permission_mask` (`number`) what scriptlet permissions should be granted to rules from the list?
+  The bits of the bitmask are currently assigned as follows:
+    - Bit `0`: "trusted" resources from the uBlock Origin project
+    - Bit `1`: resources only intended for use by Brave-authored lists
+    - Bits `2`-`7`: reserved
 
 ## Adding a new list