doc: Add 0.21.0 release notes (#1461)

doc/developer-guide/releasenotes/rel_0.21.0.rst

@@ -0,0 +1,67 @@
+.. _rel-0.21.0:
+
+Release 0.21.0
+==============
+
+Welcome to Zotonic 0.21.0, released on 3 October, 2016.
+
+Main changes are:
+
+* Added an ``acl_is_owner`` notification (:issue:`1404`).
+* Added a Docker image for Zotonic development (:issue:`1425`).
+* Improved Docker images size by switching to Alpine (:issue:`1374`).
+* Improved password hashing by switching to bcrypt (:issue:`1390`).
+* Improved ``is_visible`` filter to support fulltext search results.
+* Moved documentation to Read the Docs (:issue:`1454`).
+* Fixed removed observers not being detached (:issue:`1441`).
+
+Commits since 0.20.0
+--------------------
+
+There were 31 commits since release 0.20.0.
+
+Big thanks to all people contributing to Zotonic!
+
+Git shortlog
+............
+
+Arjan Scherpenisse (1):
+    * core: Add default config for 'setup' application
+
+David de Boer (12):
+    * core: Remove rid from maybe_allowed (#1400)
+    * tests: Fix m_identity_tests on 0.x (#1415)
+    * Add 0.20.0 release notes (#1423)
+    * mod_admin: Fix spelling error
+    * doc: Add deeplink to custompivot
+    * doc: Fix ref
+    * docker: Negate paths in .dockerignore to make image even smaller (#1426)
+    * docker: Fix compilation by switching bcrypt dep (#1434)
+    * doc: Improve site creation (fix #1440)
+    * docker: Add dev Docker image (#1425)
+    * docker: Fix duplicate config files (#1442)
+    * doc: Build 0.x docs on Read the Docs (#1454)
+
+Maas-Maarten Zeeman (1):
+    * core: Use bcrypt and erlpass for password hashes (#1390)
+
+Marc Worrell (16):
+    * mod_acl_user_groups: make the is_owner check a notification (#1404)
+    * mod_acl_user_groups: fix is_owner check.
+    * mod_admin: pass args to connect dialog items. Add 'thumbnail-linkable' class.
+    * mod_admin: in connect-dialog search result, add the class 'unpublished' for unpublished items.
+    * mod_admin: filter connect-list on visibility
+    * mod_base: let is_visible filter also accept a list of tuples {Id,Score} as returned by the fulltext search
+    * core: add smtp headers from the #email message.
+    * mod_video: fix replacement of temp re-render placeholder.
+    * i18n: manual merge of #1437
+    * mod_oembed: don't embed data for 'link' type (our metadata sniffer is better at this)
+    * mod_email_status: export clear_status/2
+    * core: fix eacces/eaccess mixup
+    * core: fix a problem where removing an exported observe function did not detach the observer. Fixes #1441
+    * core: less restrictive receive for module shutdown.
+    * core: skip identify errors when extracting mime format.
+    * mod_export: catch errors in xlsx export of illegal dates
+
+Marco Wessel (1):
+    * docker: Use Alpine Linux for the Docker images (#1374)