Add shell_command component doc

source/components/shell_command.markdown

@@ -0,0 +1,24 @@
+---
+layout: page
+title: "Shell command component"
+description: "Instructions how to integrate Sehll commands into Home Assistant."
+date: 2015-10-13 19:10
+sidebar: false
+comments: false
+sharing: true
+footer: true
+---
+
+<img src='/images/supported_brands/utilities-terminal.png' class='brand pull-right' />
+This component can expose regular shell commands as services
+
+```yaml
+# Example configuration.yaml entry
+shell_command:
+  restart_pow: touch ~/.pow/restart.txt
+```
+
+Configuration variables:
+
+- Alias for the command and the command itself.
+