Document use of fastmigrate with fastlite

Something like this:

```python
from fastlite import *
from fastmigrate import *

def get_db():

# Determine db path
if IN_NOTEBOOK or os.getenv("IN_TEST"):
dbpath = datapath/'dev_data.db'
for o in '','-shm','-wal': (datapath/f'dev_data.db{o}').unlink(missing_ok=True)
else: dbpath = datapath/'data.db'

migrations_dir = '??'

# fastmigrate: Ensure the db is versioned
ensure_versioned_db(dbpath)
# fastmigrate: run migrations
run_migrations(dbpath, migrations_dir)
# fastlight: instantiate db path
db = database(dbpath)
return db
```