docs: CEC.md, which PCs have HDMI-CEC and how to add it

CHANGELOG.md

@@ -14,6 +14,8 @@ one per merged pull request.
     the TV turns on and off with the PC. On a Steam Machine, steamos-manager
     configures it from Steam's settings.
   - Works with any `/dev/cec*` (GPU or USB adapter); the menu lists them.
+  - `CEC.md`: which PCs have CEC (NVIDIA doesn't, AMD/Intel only over some
+    DisplayPort adapters), and USB adapters for your own build.
   - Restarts steamos-manager, which only offers Steam its HDMI-CEC settings
     when cecd was there at its start.
   - The README no longer says Steam Machine support alone gives HDMI-CEC: