Update CHANGELOG to reflect v16.1 cherry-pick

Update the CHANGELOG in the main branch with the v16.1
cherrypick of r.android.com/1730332 .
Also update docs to mention updating the CHANGELOG.

Change-Id: Ib16766299000bc4ad60ed0259295024f0a9aa03c
diff --git a/CHANGELOG b/CHANGELOG
index c01f292..ccb4b8d 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -9,6 +9,12 @@
     * Added support for writing track events using custom clock timestamps.
 
 
+v16.1 - 2021-06-08:
+  Tracing service and probes:
+    * Cherry-pick of r.android.com/1716718 which missed the v16 branch cut and
+      fixed MSVC 2019 builds.
+
+
 v16.0 - 2021-06-01:
   Tracing service and probes:
     * Added support for building most targets (including traced, SDK and