Update README - Migrate Instructions

I don't know if I'm doing something wrong, but every netbox plugin I've come across since the plugin architecture was released recently seems to skip the step about running the migrations in their docs.

Author: ndom91

README.md

@@ -35,6 +35,12 @@ PLUGINS = ["netbox_onboarding"]
 # }
 ```
 
+Finally, make sure to run the migrations for this plugin
+
+```bash
+python3 manage.py migrate
+```
+
 The plugin behavior can be controlled with the following list of settings
 
 - `create_platform_if_missing` boolean (default True), If True, a new platform object will be created if the platform discovered by netmiko do not already exist and is in the list of supported platforms (`cisco_ios`, `cisco_nxos`, `arista_eos`, `juniper_junos`, `cisco_xr`)