Add fedora and alpine release builds

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

UPGRADE.md

@@ -1,5 +1,26 @@
 # Upgrading from 2.0 to 2.1
 
+## Distro-specific packages
+
+Elixir releases are precompiled binaries and are therefore dependent on the host they've been compiled to be compatible with your system (architecture, libc version, …) ([read more](https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-requirements)). For instance, the release build on Debian Bullseye (with libc 2.31) cannot be run on Debian Buster (which only provides libc 2.28). Another example is Red Hat-relative distributions (Fedora, CentOS, …) which have an OpenSSL library version that's not compatible with the version compiled (because of [potential patent issues with Elliptic curves](https://github.com/kerl/kerl#compiling-crypto-on-red-hat-systems)).
+
+In order to make sure the releases package we redistribute , we now build distro-version-specific packages.
+The list of packages built for version 2.1.0 with the amd64 (x86_64) architecture is the following:
+  - Debian Buster (10)
+  - Debian Bullseye (11)
+  - Ubuntu Bionic (18.04)
+  - Ubuntu Focal (20.04)
+  - Fedora 35
+  - Fedora 36
+  - Alpine 3.14.5
+  - Alpine 3.15.3
+
+As building for non-`amd64` architectures is quite difficult (because of the time and extra resources cross-compiling takes), these architectures are only built against Debian Bullseye (11) at the moment.
+
+We want to expand this list in the future to make sure it covers most uses, so feel free to give us feedback from what you need.
+
+Note: Docker images are also built for `amd64`/`arm`/`arm64` architectures, and of course you can always install Mobilizon from source on pretty much any unix-compatible system.
+
 ## Mailer library change
 
 ### Docker