Added a README to each of the plugins, giving an overview on how they work

plugins/shell/README.md

@@ -0,0 +1,18 @@
+# Shell
+
+Run shell commands.
+
+## Usage
+
+Type in `<prefix><command>`, where `<prefix>` is the configured prefix (default in [Configuration](#Configuration)) and `<command>` is the command you want to run.
+
+## Configuration
+
+```ron
+// <Anyrun config dir>/shell.ron
+Config(
+  prefix: ":sh",
+  // Override the shell used to launch the command
+  shell: None,
+)
+```