Readme: add nightly testing

Signed-off-by: Vasiliy Doylov <nekocwd@mainlining.org>

README.md

@@ -7,6 +7,16 @@ CallPipe - Switch call audio devices
 
 Switch voice routing via alsactl according to current primary sound device
 
+## PostmarketOS nightly testing:
+1) [Add my nightly repo](https://git.nekocwd.duckdns.org/Administration/Documentation/src/branch/master/install-nightly-repo.md)
+2) [Install Q6MeowD](https://git.nekocwd.duckdns.org/NekoCWD/q6meowd#postmarketos-nightly-testing)
+3) Install package
+  ```
+  sudo apk add callpipe@nightly
+  ```
+4) Reboot
+
+
 Project TODO:
   - [ ] Write proof of concept
     - [ ] Speaker select