Document how to uninstall

Issue #59.

README.md

@@ -53,6 +53,11 @@ git submodule update --init --recursive
 
 This will also satisfy the build requirement `inih` by pulling it in as a git submodule.
 
+To uninstall:
+```systemctl --user stop gamemoded.service
+cd build/
+ninja uninstall```
+
 ---
 ## Requesting GameMode