Added `make migrations` command to README
Added `make migrations` command to README

The instructions skipped this step to it was not possible to successfully run
the command `python3 manage.py createsuperuser` as the tables in the sqlite3
database were not complete.

pbanaszkiewicz/amy,vahtras/amy,wking/swc-amy,swcarpentry/amy,shapiromatron/amy,vahtras/amy,wking/swc-amy,pbanaszkiewicz/amy,shapiromatron/amy,swcarpentry/amy,vahtras/amy,wking/swc-amy,swcarpentry/amy,shapiromatron/amy,pbanaszkiewicz/amy,wking/swc-amy