added a note "sd has been removed" in Release note

Author: purna135

RELEASE-NOTES.md

@@ -96,6 +96,7 @@ All of the above apply to:
 This includes API changes we did not warn about since at least `3.11.0` (2021-01).
 
 - Setting initial values through `pm.Distribution(testval=...)` is now `pm.Distribution(initval=...)`.
+- In favour of `sigma`, the deprecated `sd` has been removed.
 
 
 ### New features