Windows on flask db migrate: (sqlite3.OperationalError) unable to open database file

### Problem

Trying to build the app on Windows 10.

After creating a `.env` file from `.env.example`, I tried to initialize the project from the command line like so:

```
pip install -r requirements/dev.txt
npm install
for /f "tokens=*" %i in (.env) do set %i
flask db init
flask db migrate -m "initial"
```

At the `migrate` line, there is a long stack trace, and then this error (even when running the prompt as Administrator):

```
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) unable to open database file
(Background on this error at: http://sqlalche.me/e/13/e3q8)
```

### Proposed Solution

Changing the value of `DATABASE_URL` in `.env` from the example default `sqlite:////tmp/dev.db` to a path I knew I had write access to (in my case, the project directory itself: `sqlite:///C:\\Users\\\\chime2\\dev.db`) fixed the issue.

Because the example path assumes a Linux filesystem, can a note be made, maybe in the README, that the value would need to be changed prior to building?