all: release v0.20.0

This commit will be tagged v0.20.0.

Change-Id: I99aa00c22dc772ce5ad677bcb985fb2ce89a95a8
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/52751
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Noah Dietz <ndietz@google.com>

CHANGES.md

diff --git a/CHANGES.md b/CHANGES.md
index c020684..997a22b 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,14 @@
+# v0.20.0
+
+- WithGRPCConnectionPool is a no-op for some APIs
+
+- correctly report Go version of runtime
+
+- Various updates to autogenerated clients.
+
+_Please note:_ the release version is not indicative of an individual client's
+stability or version.
+
 # v0.19.0
 
 - Various updates to autogenerated clients.