Documented new API username requirements

[kstrauser]

As of the most recent bridge software update (and perhaps earlier but I can't confirm), usernames seem to require 10 characters.

    >>> from bulby.client import HueBridgeClient
    >>> client = HueBridgeClient(username='123456789')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/private/tmp/bulby/bulby/client.py", line 44, in __init__
        self.connect()
      File "/private/tmp/bulby/bulby/client.py", line 94, in connect
        raise Exception(msg)
    Exception: invalid value,  123456789, for parameter, username

    >>> client = HueBridgeClient(username='1234567890')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/private/tmp/bulby/bulby/client.py", line 44, in __init__
        self.connect()
      File "/private/tmp/bulby/bulby/client.py", line 94, in connect
        raise Exception(msg)
    Exception: Please press the link button and try again

I updated the default username to bulbyappdev to be longer than 10 characters, and made a node in README.rst as a pointer to other devs.