add privilege pallet readme

frame/privilege/README.md

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+# Privilege
+
+This pallet implements the privilege traits.
+It currently consist of a simple storage, mapping accounts/groups to their privileges/members.
+It can be wrapped later in a privilege-governance pallet that would redirect calls to votes,
+or any other pallet which would operate filter on the privilege traits.
+
+Arbitrary implementation behavior:
+- Assuming we have a group A enforcing a privilege P, adding a user X to A is impossible (P not held by A after the operation).
+- Assuming we have a group A enforcing a privilege P, revoking a user X from A is not going to revoke his privilege P.
+
+An alternative implementation would be that promoting/revoking a user in/from a group automatically adjust its privileges.