Update repology paragraph.

Move definitions of what this method provides here.

Use `apt` and not `apt-get`, because `apt` has been the recommended
packaging interface since before Debian 9 (stretch), and calling `apt`
on ancient systems will fail--I believe this is what we want, because
a later paragraph says "PLEASE DO NOT INSTALL" ancient versions :-)

Author: sten0

README.md

@@ -41,7 +41,7 @@ With GNU Emacs 24 or later then you can use its [package][] feature (or [Cask][]
 
 ### Install using OS package system
 
-PHP Mode is available from [package systems provided by several OSs](https://repology.org/project/emacs:php-mode/versions).  This is the easiest way to add PHP to Emacs and can be installed with just `sudo apt-get install elpa-php-mode`.
+PHP Mode is available from [package systems provided by several OSs](https://repology.org/project/emacs:php-mode/versions).  On Debian, and operating systems derived from it, such as Ubuntu, the easiest way to add PHP support to Emacs is `sudo apt install elpa-php-mode`.  "Stable" releases of these operating systems include a well-tested frozen version of this package that is always older than the latest upstream version of PHP Mode.  Autoloads and byte-compilation are automatic.
 
 The package name on Debian, Ubuntu, and some derived operating systems is [`elpa-php-mode`](https://packages.debian.org/buster/elpa-php-mode).  The packages provided for these operating systems are stable, but since they are behind the latest version of PHP Mode, consider the possibility of not incorporating the latest PHP feature support and performance improvements.