Add to README.md how to run the tests

[Pander]

Please, add to README.md how to run the tests.

[Grey Li]

#171 just update the development section in the docs. I will add a contributing guide later.

[Pander]

Thanks. Add also at https://github.com/greyli/bootstrap-flask/blame/master/README.md#L25

Install development version with:

```
$ python setup.py install
```

[Pander]

Tests can be run with:
```
$ pytest
```
or for more verbose output and stop after first failure:
```
$ pytest -vx
```