Document how to set admin password in web2py parameters_PORTNUM.py file.

[kfogel]

In README.md, we document a slightly awkward method for setting the initial admin password. We should also document that it can be directly set in the password option in parameters_PORTNUM.py file in the web2py root, although one needs to be careful to give the appropriate prefix to the value to specify the hashing method of course. (There's apparently some code to do it in scripts/setup-*.py.)