README: specify the meaning of "not too ancient Linux"

Commits like 3b19866882 show that in
practice, only the lastest releases of popular distributions are
supported.

README.md

@@ -36,7 +36,8 @@ Releases can be found on the [release list][releases].
 
 ## System requirements
 
-- A not too ancient Linux, Windows 10 or later, or macOS 10.15 or later.
+- A not too ancient Linux (usually, only the latest releases of distributions
+  are actively supported), Windows 10 or later, or macOS 10.15 or later.
 - A somewhat capable CPU. Hardware decoding might help if the CPU is too slow to
   decode video in realtime, but must be explicitly enabled with the `--hwdec`
   option.