docs: Add insiders note to readme

pawamoy · pawamoy · commit 1dcbba53899c · 2023-07-15T18:57:06.000+02:00
diff --git a/README.md b/README.md
@@ -0,0 +1,4 @@
+# griffe-typing-deprecated
+
+This project is currently available to [sponsors](https://github.com/sponsors/pawamoy) only.
+See the documentation here: https://pawamoy.github.io/griffe-typing-deprecated.
