Documentation Update: When Upgrading/Installing Run Migrations

[jvanderaa]

Environment

• Python version: Python 3.7.7
• NetBox version: 2.8.9
• ntc-netbox-plugin-onboarding version: 2.0.0

Steps to Reproduce

1. Change the pip install from 1.3 to 2.0
2. Start server
3. Check on devices, attempt to use the API

Expected Behavior

Everything should be working in a normal state

Observed Behavior

500 errors when using the API and programming errors when navigating to a device in NetBox

Resolution

Run migrations after installing the update

[dgarros]

I think it's already mentioned in the readme (Line 54)
Maybe should add a brief section about upgrade