local.d: update README

Remind users that if they put a long-running process in /etc/local.d,
they can delay their boot or shutdown processing.

Reported-by: toralf.foerster@gmx.de
X-Gentoo-Bug: 456650
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=456650

local.d/README

@@ -7,3 +7,8 @@ executable and it has a .stop extension, it will be run when the local
 service is stopped.
 
 All files are processed in lexical order.
+
+Keep in mind that files in this directory are processed sequencially,
+and the local service is not considered started or stopped until
+everything is processed, so if you have a process which takes a long
+time to run, it can delay your boot or shutdown processing.