Fix README markdown

When attempting to run counterpartyd.py:

```
$ python counterpartyd.py       
Traceback (most recent call last):
  File "counterpartyd.py", line 21, in <module>
    from lib import config, api, util, exceptions, bitcoin, blocks, blockchain
  File "/home/u/counterparty/counterpartyd/lib/api.py", line 16, in <module>
    import apsw
ImportError: No module named 'apsw'
```

If this dependency was deliberately left out of pip-requirements.txt, then please disregard this PR. Also includes cosmetic touchup to README.md