Add mautrix-signal appservice instructions

APPSERVICES.md

@@ -7,9 +7,32 @@ If you run into any problems while setting up an Appservice, write an email to `
 ## Tested appservices
 
 Here are some appservices we tested and that work with Conduit:
-- matrix-appservice-discord
+- [matrix-appservice-discord](https://github.com/Half-Shot/matrix-appservice-discord)
-- mautrix-hangouts
+- [mautrix-hangouts](https://github.com/mautrix/hangouts/)
-- mautrix-telegram
+- [mautrix-telegram](https://github.com/mautrix/telegram/)
+- [mautrix-signal](https://github.com/mautrix/signal)
+  - There are a few things you need to do, in order for the bridge (at least up to version `0.2.0`) to work. Before following the bridge installation guide, you need to map apply a patch to bridges `portal.py`. Go to [portal.py](https://github.com/mautrix/signal/blob/master/mautrix_signal/portal.py) at [mautrix-signal](https://github.com/mautrix/signal) (don't forget to change to the correct commit/version of the file) and copy its content, create a `portal.py` on your host system and paste it in. Now you need to change two lines:
+  [Line 1020](https://github.com/mautrix/signal/blob/4ea831536f154aba6419d13292479eb383ea3308/mautrix_signal/portal.py#L1020)
+
+    ```diff
+    --- levels.users[self.main_intent.mxid] = 9001 if is_initial else 100
+    +++ levels.users[self.main_intent.mxid] = 100 if is_initial else 100
+    ```
+
+    and add a new line between [Lines 1041 and 1042](https://github.com/mautrix/signal/blob/4ea831536f154aba6419d13292479eb383ea3308/mautrix_signal/portal.py#L1041-L1042)
+
+    ```diff
+        "type": str(EventType.ROOM_POWER_LEVELS),
+    +++ "state_key": "",
+        "content": power_levels.serialize(),
+    ```
+
+    Now you just need to map the patched `portal.py` into the `mautrix-signal` container
+    ```yml
+    volumes:
+      - ./<your>/<path>/<on>/<host>/portal.py:/usr/lib/python3.9/site-packages/mautrix_signal/portal.py
+    ```
+    and then read below and start following the bridge [installation instructions](https://docs.mau.fi/bridges/index.html).
 
 ## Set up the appservice