Doc: ChangeLog: update for 1.1.21-rc1 release

ChangeLog

@@ -1,3 +1,19 @@
+* Wed May 22 2019 Ken Gaillot <[email protected]> Pacemaker-1.1.21-rc1
+- Changesets: 64
+- Diff:       39 files changed, 1985 insertions(+), 843 deletions(-)
+
+- Changes since Pacemaker-1.1.20
+  + Important security fixes for CVE-2018-16878, CVE-2018-16877, CVE-2019-3885
+  + tools: ensure crm_resource --clean-up works with multiple failures
+    (regression in 1.1.20)
+  + build: crm_report bug report URL is now configurable at build time
+  + crmd: avoid memory leak when duplicate monitor is scheduled
+  + pengine: respect order constraints when resources are being probed
+  + pengine: one group stop shouldn't make another required
+  + pengine: silence log message about symmetrical property of serialize orders
+  + libcrmcommon: handle out-of-range integers in configuration better
+  + libcrmservice: fix use-after-free memory error in alert handling
+
 * Mon Mar 04 2019 Ken Gaillot <[email protected]> Pacemaker-1.1.20-1
 - Changesets: 303
 - Diff:       139 files changed, 8751 insertions(+), 4923 deletions(-)