Add 2.1.1 relnotes to RELEASE.md

RELEASE.md

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+# Release 2.2.1
+
+## TensorBoard.dev updates
+
+- TensorBoard.dev now renders model graphs, for experiments uploaded starting
+  from this release.
+  - The `tensorboard dev upload` subcommand now sends the model graph, when
+    available, so that it can be rendered via the Graphs plugin on
+    TensorBoard.dev.
+  - Large node attribute values (which would not be rendered anyway) are
+    filtered out before upload.
+  - Graphs that remain larger than 10MB after filtering are not uploaded.
+- The `tensorboard dev upload` command supports a `--plugins` option to
+  explicitly indicate the desired plugins for which summary data should be
+  uploaded (#3402, #3492)
+- The `tensorboard dev list` subcommand now reports the total size of stored
+  binary objects (e.g., graphs) for each experiment (#3464)
+- The `tensorboard dev list` subcommand now accepts a `--json` flag to allow
+  parsing the output more easily (#3480)
+
+## Features
+
+- Auto-reload is now disabled when the browser tab is not visible, saving
+  network bandwidth (#3483)
+- New logo used in the favicon (#3406)
+
+## Bug fixes
+
+- Plugin loading: When a plugin fails to load, TensorBoard logs an error and
+  continues, instead of crashing (#3484, #3486)
+- Eliminated sporadic HTTP 500 errors for XHRs that do markdown rendering (#3491)
+
 # Release 2.2.0
 
 The 2.2 minor series tracks TensorFlow 2.2.