Add Linux installation info (k4zmu2a#103)

* Add Linux installation info

Add a repology badge and mention the possibility to install via the package manager.

I believe this is a better approach than explicitly mentioning a repository, because it uses repology's database (and the list will remain updated).

* use tiny badge

The big badge is too big and could overflow in the following section. The tiny one will keep the same size as new packages are added.

README.md

@@ -63,6 +63,10 @@ Install devel packages for `SDL2` and `SDL2_mixer`.\
 Compile with CMake; tested with GCC 10, Clang 11.\
 To cross-compile for Windows, install a 64-bit version of mingw and its `SDL2` and `SDL2_mixer` distributions, then use the `mingwcc.cmake` toolchain.
 
+[![Packaging status](https://repology.org/badge/tiny-repos/spacecadetpinball.svg)](https://repology.org/project/spacecadetpinball/versions) 
+
+Some distributions provide a package in their repository. You can use those for easier dependency management and updates.
+
 ### On macOS
 
 * **Homebrew**: Install the `SDL2`, `SDL2_mixer` homebrew packages.