chore: add docs about installing master branch

docs/source/troubleshooting/install-master.md

@@ -0,0 +1,13 @@
+# Install master
+
+HACS no longer supports installing from the master branch.
+If no new version of PowerCalc has been released yet and you need to test the latest functionality, you can update manually by following these steps:
+
+1. Download the ZIP file from this URL: https://github.com/bramstroker/homeassistant-powercalc/archive/refs/heads/master.zip
+2. Extract the contents of the ZIP.
+3. Copy the custom_components/powercalc folder from the ZIP into your Home Assistant's config/custom_components/powercalc directory, replacing the existing files.
+4. Restart Home Assistant to apply the latest improvements and updates.
+
+!!! note
+
+    Don't worry, next time a new version is released and you install it, HACS will update PowerCalc to the latest version, overwritting your temporary modifications.