Add OxiPNG packaging status infographic powered by Repology

Repology is a free service that monitors the packaging status of
projects in lots of package repositories, and offers a nice API to
generate badges with a summary of what OxiPNG versions are available in
what repositories. Let's introduce that to our README to make it easier
for users to decide whether to use a package manager of their
convenience, and visibilize the work package maintainers do with OxiPNG.

README.md

@@ -14,7 +14,10 @@ interface or as a library in other Rust programs.
 
 Oxipng for Windows can be downloaded from the [Releases](https://github.com/shssoichiro/oxipng/releases) link on the GitHub page.
 
-For MacOS or Linux, it is recommended to install from your distro's package repository, if possible.
+For MacOS or Linux, it is recommended to install from your distro's package
+repository, if possible. Oxipng is known to be packaged for the environments listed below.
+
+[![Packaging status](https://repology.org/badge/vertical-allrepos/oxipng.svg?exclude_unsupported=1&columns=3&exclude_sources=modules,site)](https://repology.org/project/oxipng/versions)
 
 Alternatively, oxipng can be installed from Cargo, via the following command: