Added changelog entry for 3.12.1.

CHANGES.txt

@@ -1,3 +1,10 @@
+2020-05-20 version 3.12.1 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
+
+  Ruby
+  * Re-add binary gems for Ruby 2.3 and 2.4. These are EOL upstream, however
+    many people still use them and dropping support will require more
+    coordination.
+
 2020-05-12 version 3.12.0 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
   Protocol Compiler