doc: Add section about supported distribution images

Having a list of supported distributions in the manual has been long
overdue. Complementing it with the expected formats should make lives of
users a bit easier.

containers#977

Author: martymichal

doc/toolbox.1.md

@@ -40,6 +40,20 @@ access to the user's home directory, the Wayland and X11 sockets, networking
 (including Avahi), removable devices (like USB sticks), systemd journal, SSH
 agent, D-Bus, ulimits, /dev and the udev database, etc..
 
+## Supported distribution images
+
+Toolbox maintains a list of supported distribution images. These images are
+maintained by the team to be compatible with Toolbox. Some commands accept the
+`--distro` and `--release` flags. `--distro` accepts `ID` from `os-release`
+(e.g., on Fedora it is `fedora`). `--release` accepts version number of the
+selected distribution. Each distribution uses sligthly different versioning
+schema and some don't have any at all. Supported distributions are:
+
+Name  |os-release|Format                     |URI
+------|----------|---------------------------|-----------------------------------------
+Fedora|fedora    |\<release\> or f\<release\>|registry.fedoraproject.org/fedora-toolbox
+RHEL  |rhel      |\<major\>.\<minor\>        |registry.access.redhat.com/ubi8/ubi
+
 ## GLOBAL OPTIONS ##
 
 The following options are understood: