Add release notes for v0.21.0 (#6203)

* Add release notes for v0.21.0

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update doc/whats-new.rst

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

HOW_TO_RELEASE.md

@@ -25,11 +25,11 @@ upstream        https://github.com/pydata/xarray (push)
      ```
  3. Add a list of contributors with:
     ```sh
-    git log "$(git tag --sort="v:refname" | tail -1).." --format=%aN | sort -u | perl -pe 's/\n/$1, /'
+    git log "$(git tag --sort=v:refname | tail -1).." --format=%aN | sort -u | perl -pe 's/\n/$1, /'
     ```
     This will return the number of contributors:
     ```sh
-    git log $(git tag --sort="v:refname" | tail -1).. --format=%aN | sort -u | wc -l
+    git log "$(git tag --sort=v:refname | tail -1).." --format=%aN | sort -u | wc -l
     ```
  4. Write a release summary: ~50 words describing the high level features. This
     will be used in the release emails, tweets, GitHub release notes, etc.

doc/whats-new.rst

@@ -14,10 +14,19 @@ What's New
 
     np.random.seed(123456)
 
-.. _whats-new.0.X.Y+1:
+.. _whats-new.0.21.0:
+
+v0.21.0 (27 January 2022)
+-------------------------
+
+Many thanks to the 20 contributors to the v0.21.0 release!
+
+Abel Aoun, Anderson Banihirwe, Ant Gib, Chris Roat, Cindy Chiao,
+Deepak Cherian, Dominik Stańczak, Fabian Hofmann, Illviljan, Jody Klymak, Joseph
+K Aicher, Mark Harfouche, Mathias Hauser, Matthew Roeschke, Maximilian Roos,
+Michael Delgado, Pascal Bourgault, Pierre, Ray Bell, Romain Caneill, Tim Heap,
+Tom Nicholas, Zeb Nicholls, joseph nowak, keewis.
 
-v0.21.0 (unreleased)
----------------------
 
 New Features
 ~~~~~~~~~~~~
@@ -65,10 +74,6 @@ Bug fixes
   By `Pierre Loicq <https://github.com/pierreloicq>`_.
 - Determination of zarr chunks handles empty lists for encoding chunks or variable chunks that occurs in certain cirumstances (:pull:`5526`). By `Chris Roat <https://github.com/chrisroat>`_.
 
-Documentation
-~~~~~~~~~~~~~
-
-
 Internal Changes
 ~~~~~~~~~~~~~~~~