• Added: Support for rfc6638, also known as CalDAV Scheduling.
• Added: Automatically converting between vCard 3, 4 and jCard using the
  Accept: header, in CardDAV reports, and automatically converting from
  jCard to vCard upon PUT. It's important to note that your backends may
  now recieve both vCard 3.0 and 4.0.
• Added: #444. Collections can now opt-in to support high-speed MOVE.
• Changed: PropertyStorage backends now have a move method.
• Added: beforeMove, and afterMove events.
• Changed: A few database changes for the CalDAV PDO backend. Make sure you
  run bin/migrate21.php to upgrade your database schema.
• Changed: CalDAV backends have a new method: getCalendarObjectByUID. This
  method MUST be implemented by all backends, but the AbstractBackend has a
  simple default implementation for this.
• Changed: Sabre\CalDAV\UserCalendars has been renamed to
  Sabre\CalDAV\CalendarHome.
• Changed: Sabre\CalDAV\CalendarRootNode has been renamed to
  Sabre\CalDAV\CalendarRoot.
• Changed: The IMipHandler has been completely removed. With CalDAV scheduling
  support, it is no longer needed. It's functionality has been replaced by
  Sabre\CalDAV\Schedule\IMipPlugin, which can now send emails for clients
  other than iCal.
• Removed: Sabre\DAV\ObjectTree and Sabre\DAV\Tree\FileSystem. All this
  functionality has been merged into Sabre\DAV\Tree.
• Changed: PrincipalBackend now has a findByUri method.
• Changed: PrincipalBackend::searchPrincipals has a new optional test
  argument.
• Added: Support for the {http://calendarserver.org/ns/}email-address-set
  property.
• #460: PropertyStorage must move properties during MOVE requests.
• Changed: Restructured the zip distribution to be a little bit more lean
  and consistent.
• #524: Full support for the test="anyof" attribute in principal-search
  REPORT.
• #472: Always returning lock tokens in the lockdiscovery property.
• Directory entries in the Browser plugin are sorted by type and name.
  (@aklomp)
• #486: It's now possible to return additional properties when an 'allprop'
  PROPFIND request is being done. (@aklomp)
• Changed: Now return HTTP errors when an addressbook-query REPORT is done
  on a uri that's not a vcard. This should help with debugging this common
  mistake.
• Changed: PUT requests with a Content-Range header now emit a 400 status
  instead of 501, as per RFC7231.
• Added: Browser plugin can now display the contents of the
  {DAV:}supported-privilege-set property.
• Added: Now reporting CALDAV:max-resource-size, but we're not actively
  restricting it yet.
• Changed: CalDAV plugin is now responsible for reporting
  CALDAV:supported-collation-set and CALDAV:supported-calendar-data
  properties.
• Added: Now reporting CARDDAV:max-resource-size, but we're not actively
  restricting it yet.
• Changed: CalDAV plugin is now responsible for reporting
  CALDAV:supported-collation-set and CALDAV:supported-calendar-data
  properties.
• Added: Now reporting CARDDAV:max-resource-size, but we're not actively
  restricting it yet.
• Added: Support for CARDDAV:supported-collation-set.
• Changed: CardDAV plugin is now responsible for reporting
  CARDDAV:supported-address-data. This functionality has been removed from
  the CardDAV PDO backend.
• When a REPORT is not supported, we now emit HTTP error 415, instead of 403.
• #348: HEAD requests now work wherever GET also works.
• Changed: Lower priority for the iMip plugins schedule event listener.
• Added: #523 Custom CalDAV backends can now mark any calendar as read-only.
• The zip release ships with sabre/vobject 3.3.3,
  sabre/http 3.0.0, and sabre/event 2.0.0.