Add docs to tokenfactory (backport #6095)

x/tokenfactory/README.md

@@ -17,6 +17,42 @@ created denom. Once a denom is created, the original creator is given
   account, or even setting it to `""`, meaning no account has admin privileges
   of the asset.
 
+## Bank hooks
+Token factory supports better integration with contracts using bank hooks.
+
+Token factory is integrated with Before Send bank hooks, `TrackBeforeSend` and `BlockBeforeSend`. Both hooks gets called whenever a bank send takes place, the difference between two hooks is that `TrackBeforeSend` would not error and `BlockBeforeSend` errors. Due to this difference `TrackBeforeSend` is useful for cases when a contract needs to track specific send actions of the token factory denom, whilst `BlockBeforeSend` would be more useful for situations when we want to block specific sends using contracts.
+
+Each Token Factory denom allows the registration of one contract address. This contract is sudo-called every time the aforementioned bank hooks are activated. 
+
+Contracts are able to integrate with these hooks by implementing `BlockBeforeSend` and `TrackBeforeSend` message as the following example: 
+
+```rust
+#[entry_point]
+pub fn sudo(deps: DepsMut, env: Env, msg: SudoMsg) ->  StdResult<Response> {
+    match &msg{
+        SudoMsg::BlockBeforeSend { from, to, amount} => {
+            Ok(Response::new().add_attributes(vec![
+                ("hook", "block"),
+                ("from", from),
+                ("to", to),
+                ("amount", &amount.to_string())
+            ]))
+        },
+        SudoMsg::TrackBeforeSend { from, to, amount} => {
+            Ok(Response::new().add_attributes(vec![
+                ("hook", "track"),
+                ("from", from),
+                ("to", to),
+                ("amount", &amount.to_string())
+            ]))
+        }
+    }
+}
+```
+
+
+Note that since `TrackBeforeSend` hook can also be triggered upon module to module send (which is not gas metered), we internally gas meter `TrackBeforeSend` with a gas limit of 100_000. 
+
 ## Messages
 
 ### CreateDenom