README: installing custom scripts & modules

(cherry picked from commit ecde864263)

README.d/hello-world.rsc

@@ -0,0 +1,3 @@
+#!rsc by RouterOS
+
+:put ("Hello World from " . [ / system identity get name ] . "!");

README.md

@@ -246,6 +246,29 @@ Available modules
 * [Send notifications via Telegram](doc/mod/notification-telegram.md)
 * [Download script and run it once](doc/mod/scriptrunonce.md)
 
+Installing custom scripts & modules
+-----------------------------------
+
+My scripts cover a lot of use cases, but you may have your own ones. You can
+still use my scripts to manage and deploy yours, by specifying `base-url`
+(and `url-suffix`) for each script.
+
+This will fetch and install a script `hello-world.rsc` from the given url:
+
+    $ScriptInstallUpdate hello-world.rsc "base-url=https://git.eworm.de/cgit/routeros-scripts/plain/README.d/"
+
+![screenshot: install custom script](README.d/12-install-custom-script.avif)
+
+(Yes, the example url still belongs to the repository for easy
+handling - but the url can be what ever you use.)
+
+For a script to be considered valid it has to begin with a *magic token*.
+Have a look at [any script](README.d/hello-world.rsc) and copy the first line
+without modification.
+
+Starting a script's name with `mod/` makes it a module and it is run
+automatically by `global-functions`.
+
 Contact
 -------