Withdraw this explainer

Thank you all for feedback in many channels.  Please redirect to the [TURTLEDOVE explainer](https://github.com/michaelkleber/turtledove) for a more private way to address this use case.

README.md

@@ -1,4 +1,8 @@
-# PIGIN: Private Interest Groups, Including Noise
+## PIGIN: Private Interest Groups, Including Noise
+
+# **This proposal has been withdrawn —**
+# **Please see its successor, [TURTLEDOVE](https://github.com/michaelkleber/turtledove)**
+
 
 ## Introduction