doc: update README with GNU/Musl description.

Author: flavorjones

README.md

@@ -10,6 +10,43 @@ This is kind of successor of [rake-compiler-dev-box](https://github.com/tjschuck
 It is wrapped as a gem for easier setup, usage and integration and is based on lightweight Docker containers.
 It is also more reliable, since the underlying docker images are versioned and immutable.
 
+## Supported platforms
+
+The following platforms are supported for cross-compilation by rake-compiler-dock:
+
+- `aarch64-linux` and `aarch64-linux-gnu`
+- `aarch64-linux-musl`
+- `arm-linux` and `arm-linux-gnu`
+- `arm-linux-musl`
+- `arm64-darwin`
+- `jruby`
+- `x64-mingw-ucrt`
+- `x64-mingw32`
+- `x86-linux` and `x86-linux-gnu`
+- `x86-linux-musl`
+- `x86-mingw32`
+- `x86_64-darwin`
+- `x86_64-linux` and `x86_64-linux-gnu`
+- `x86_64-linux-musl`
+
+### Windows
+
+`x64-mingw-ucrt` should be used for Ruby 3.1 and later on windows. `x64-mingw32` should be used for Ruby 3.0 and earlier.
+
+### GNU and Musl
+
+Platform names with a `*-linux` suffix are aliases for `*-linux-gnu`, since the Rubygems default is to assume `gnu` if no libc is specified.
+
+Some C extensions may not require separate GNU and Musl builds, in which case it's acceptable to ship a single `*-linux` gem to cover both platforms.
+
+The `*-linux-gnu` and `*-linux-musl` platform name suffixes require Rubygems 3.3.22 or later (or Bundler 2.3.21 or later) at installation time. Ruby version 3.0 and later ship with a sufficient Rubygems version, but versions compatible with earlier Rubies are:
+
+- ruby: "2.7", rubygems: "3.4.22"
+- ruby: "2.6", rubygems: "3.4.22"
+- ruby: "2.5", rubygems: "3.3.26"
+- ruby: "2.4", rubygems: "3.3.26"
+
+
 ## Installation
 
 Install docker [following the instructions on the docker website](https://docs.docker.com/engine/install/) ... or install [docker-toolbox for Windows and OSX](https://github.com/docker/toolbox/releases) or boot2docker on [Windows](https://github.com/boot2docker/windows-installer/releases) or [OS X](https://github.com/boot2docker/osx-installer/releases) .
@@ -33,10 +70,12 @@ Your Rakefile should enable cross compilation like so:
 ```ruby
 exttask = Rake::ExtensionTask.new('my_extension', my_gem_spec) do |ext|
   ext.cross_compile = true
-  ext.cross_platform = %w[x86-mingw32 x64-mingw-ucrt x64-mingw32 x86-linux-gnu x86_64-linux-gnu x86_64-darwin arm64-darwin]
+  ext.cross_platform = %w[x86-mingw32 x64-mingw-ucrt x64-mingw32 x86-linux x86_64-linux x86_64-darwin arm64-darwin]
 end
 ```
 
+where you should choose your platforms from the list in the "Supported platforms" section.
+
 See below, how to invoke cross builds in your Rakefile.
 
 Additionally it may also be used to build ffi based binary gems like [libusb](https://github.com/larskanis/libusb), but currently doesn't provide any additional build helpers for this use case, beyond docker invocation and cross compilers.