Document using custom authlib-injector JAR

Signed-off-by: Evan Goode <[email protected]>

README.md

@@ -100,3 +100,7 @@ AppImages are available in the [releases section](https://github.com/unmojang/Fj
 ## Building
 
 To build the launcher yourself, follow [the instructions on the Prism Launcher website](https://prismlauncher.org/wiki/development/build-instructions) but clone this repo instead.
+
+## Notes
+
+- You can easily use a custom version of authlib-injector on an instance. Select the instance in the main window, click "Edit" (or Ctrl+I/Command+I), go to the Version tab, click "Add Agents", and select your authlib-injector JAR. If your JAR is not correctly identified as authlib-injector, make sure the `Agent-Class` field in the JAR's MANIFEST.MF is `moe.yushi.authlibinjector.Premain`.