Update CHANGES for 2.20.0

Author: ricellis

CHANGES.md

@@ -1,6 +1,6 @@
-# Unreleased
-- [DEPRECATED] This library is now deprecated and will be EOL on Dec 31 2021.
+# 2.20.0 (2021-08-26)
 - [FIXED] Type of `sinceSeq` can be also a `String` besides an `Integer`.
+- [DEPRECATED] This library is now deprecated and will be EOL on Dec 31 2021.
 - [IMPROVED] - Document IDs and attachment names are now rejected if they could cause an unexpected
   Cloudant request. We have seen that some applications pass unsantized document IDs to SDK functions
   (e.g. direct from user requests). In response to this we have updated many functions to reject