new function: install shell command extension by Arksine

docs/Shell Command Extension.md

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+# Shell Command Extension
+
+### Creator of this extension is [Arksine](https://github.com/Arksine).
+
+This is a brief explanation of how to use the shell command extension for Klipper, which you can install with KIAUH.
+
+After installing the extension you can execute linux commands or even scripts from within Klipper with custom commands defined in your printer.cfg.
+
+#### How to configure a shell command:
+
+```shell
+# Runs a linux command or script from within klipper.  Note that sudo commands
+# that require password authentication are disallowed. All executable scripts
+# should include a shebang.
+# [shell_command my_shell_cmd]
+#command:
+#  The linux shell command/script to be executed.  This parameter must be
+#  provided
+#timeout: 2.
+#  The timeout in seconds until the command is forcably terminated.  Default
+#  is 2 seconds.
+#verbose: True
+#  If enabled, the command's output will be forwarded to the terminal.  Its
+#  recommended to set this to false for commands that my run in quick
+#  succession.  Default is True.
+```
+
+Once you have set up a shell command with the given parameters from above in your printer.cfg you can run the command as follows:
+`RUN_SHELL_COMMAND CMD=name`
+
+Example:
+
+```
+[shell_command hello_world]
+command: echo hello world
+timeout: 2.
+verbose: True
+```
+
+Execute with:
+`RUN_SHELL_COMMAND CMD=hello_world`
+
+## Warning
+
+This extension may have a high potential for abuse if not used carefully! Also, depending on the command you execute, high system loads may occur and can cause system instabilities.
+Use this extension at your own risk and only if you know what you are doing!