docs: Add upgrade guide and update changelog for release 3.0.0

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+## [3.0.0] - 2019-05-18
+
 ### Changed
 
 + Switched base image to Debian to make usage of OpenCV feasible
@@ -176,7 +178,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 + Initial release
 
-[Unreleased]: https://github.com/mserajnik/hydrus-server-docker/compare/2.15.0...develop
+[Unreleased]: https://github.com/mserajnik/hydrus-server-docker/compare/3.0.0...develop
+[3.0.0]: https://github.com/mserajnik/hydrus-server-docker/compare/2.15.0...3.0.0
 [2.15.0]: https://github.com/mserajnik/hydrus-server-docker/compare/2.14.0...2.15.0
 [2.14.0]: https://github.com/mserajnik/hydrus-server-docker/compare/2.13.0...2.14.0
 [2.13.0]: https://github.com/mserajnik/hydrus-server-docker/compare/2.12.0...2.13.0

README.md

@@ -12,6 +12,7 @@ The latest build runs [hydrus server version 351][hydrus-server-version].
 + [Install](#install)
   + [Dependencies](#dependencies)
   + [Updating](#updating)
+    + [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)
   + [Additional configuration when building](#additional-configuration-when-building)
@@ -63,6 +64,14 @@ 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 2.x to 3.x
+
+Upgrading from `2.x` to `3.x` can be done via pulling the updated image from
+Docker Hub or building it yourself and requires no further manual changes.
+
+Due to the introduction of OpenCV to hydrus server (which is hard to build on
+Alpine), the Docker image is now based on Debian instead of Alpine instead.
+
 #### Upgrading from 1.x to 2.x
 
 Upgrading from `1.x` to `2.x` can be done via pulling the updated image from