docs: add SPM warning

Author: robingenz

README.md

@@ -1,3 +1,6 @@
+> [!WARNING]
+> These plugins do **not** support [Swift Package Manager](https://www.swift.org/documentation/package-manager/) (SPM) because the underlying [ML Kit SDKs do not support SPM](https://github.com/googlesamples/mlkit/issues/180#issuecomment-1298964099) (see [#186](https://github.com/capawesome-team/capacitor-mlkit/issues/186)).
+
 <br />
 <div align="center">
   <img src="https://avatars.githubusercontent.com/u/105555861" width="128" height="128" />