add release docs

RELEASE.md

@@ -0,0 +1,15 @@
+# Releasing Firmware
+
+GitHub Actions is set up to automatically build and release firmware.
+
+It will automatically build firmware when one of the following tag formats are pushed.
+
+- `companion-v1.0.0`
+- `repeater-v1.0.0`
+- `room-server-v1.0.0`
+
+> NOTE: replace `v1.0.0` with the version you want to release as.
+
+- You can push one, or more tags on the same commit, and they will all build separately.
+- Once the firmware has been built, a new (draft) GitHub Release will be created.
+- You will need to update the release notes, and publish it.