Bug 1449877 - Update geckodriver changelog for 0.20.1. r=whimboo

MozReview-Commit-ID: DF36cXETqy6

--HG--
extra : rebase_source : c81ed29e49804ab43638320ace7ea118743cba8a

testing/geckodriver/CHANGES.md

@@ -3,6 +3,22 @@ Change log
 
 All notable changes to this program is documented in this file.
 
+0.20.1 (2018-04-06)
+-------------------
+
+### Fixed
+
+- Avoid attempting to kill Firefox process that has stopped.
+
+  With the change to allow Firefox enough time to shut down in
+  0.20.0, geckodriver started unconditionally killing the process
+  to reap its exit status.  This caused geckodriver to inaccurately
+  report a successful Firefox shutdown as a failure.
+
+  The regression should not have caused any functional problems, but
+  the termination cause and the exit status are now reported correctly.
+
+
 0.20.0 (2018-03-08)
 -------------------