Added guide on using docker and editing files

PluginDevDockerUsage.md

@@ -0,0 +1,16 @@
+## Setup
+1. Run `./docker.sh`
+2. Go to http://localhost:8000/ in your browser.
+3. Follow the WordPress page's setup guide create your user
+4. After logging in to the WordPress admin go to "Plugins" > "Add New" > "Upload Plugin"
+5. Zip up all file in this directory expect "docker-instance-files"
+6. Upload this on the page from step 4. above
+7. Make sure to active the plugin on the Plugins page
+8. Your done, happy editing!
+
+## Editing
+1. Test changes by editing files under docker-instance-files/plugins/
+  - Modify these directly on your host machine, changes take effect immediately!
+2. Make sure to get your changes in to the original source to commit
+  - Careful not to re-upload your plugin, it will overwrite files docker-instance-files/plugins/
+