Update README.md - remark unsupported Python features (#1161)

Since Python 3.12 is officially released it seems appropriate to remark that YAPF
does not support it currently.
diff --git a/README.md b/README.md
index c798165..5ce50b7 100644
--- a/README.md
+++ b/README.md
@@ -347,6 +347,8 @@
   --binary BINARY       location of binary to use for YAPF
 ```
 
+## Unsupported Python features
+* Python 3.12 – [PEP 701 – Syntactic formalization of f-strings](https://peps.python.org/pep-0701/) – [YAPF #1136](https://github.com/google/yapf/issues/1136)
 
 ## Knobs