docs: update readme and changelog for release 4.0.0

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+## [4.0.0] - 2020-10-22
+
 ### Changed
 
 + Updated hydrus server version
@@ -375,7 +377,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 + Initial release
 
-[Unreleased]: https://github.com/imtbl/hydrus-server-docker/compare/3.29.0...develop
+[Unreleased]: https://github.com/imtbl/hydrus-server-docker/compare/4.0.0...develop
+[4.0.0]: https://github.com/imtbl/hydrus-server-docker/compare/3.29.0...4.0.0
 [3.29.0]: https://github.com/imtbl/hydrus-server-docker/compare/3.28.0...3.29.0
 [3.28.0]: https://github.com/imtbl/hydrus-server-docker/compare/3.27.0...3.28.0
 [3.27.0]: https://github.com/imtbl/hydrus-server-docker/compare/3.26.0...3.27.0

README.md

@@ -18,6 +18,7 @@ of this schedule (e.g., important changes to hydrus server).
 + [Install](#install)
   + [Dependencies](#dependencies)
   + [Updating](#updating)
+    + [Upgrading from 3.x to 4.x](#upgrading-from-3x-to-4x)
     + [Upgrading from 2.x to 3.x](#upgrading-from-2x-to-3x)
     + [Upgrading from 1.x to 2.x](#upgrading-from-1x-to-2x)
 + [Usage](#usage)
@@ -71,6 +72,14 @@ are therefore always safe to simply install via the routine mentioned before.
 When necessary, this section will be expanded with upgrade guides to new major
 versions.
 
+#### Upgrading from 3.x to 4.x
+
+Upgrading from `3.x` to `4.x` can be done via pulling the updated image from
+Docker Hub or building it yourself and requires no further manual changes.
+
+`4.0.0` has introduced no breaking changes and merely reflects the switch to a
+new license (AGPLv3).
+
 #### Upgrading from 2.x to 3.x
 
 Upgrading from `2.x` to `3.x` can be done via pulling the updated image from