add readme, fix #1

readme.md

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+
+# shell mod for minetest
+
+Allows you to execute chat commands via text files on the server-machine
+
+## Usage
+
+After installing the mod everything you write into `shell.txt` in the world directory will be executed as chatcommand.
+The results from the last command are saved in `shell_result.txt`
+
+The executing player is currently `admin` and hardcoded (could be made into a setting)
+
+### Shutdown the server with a countdown
+
+```
+echo "shutdown 60" > shell.txt
+```
+
+### Message a player
+
+```
+echo "msg someplayer hello" > shell.txt
+```
+