Please document performance

[dimaqq]

At the first glance, pure-Python seems slow, is the underlying crypto implemented in pure Python, or does it use something from stdlib, etc...

So, for someone who doesn't want to dig too deep, README.md could have a simple performance comparison, e.g. against pypi:scrypt with same settings.

[ricmoo]

The algorithm is in Pure-Python. It will be excruciatingly slow.

In PyPy it is snappy though.

The main values in a Pure-Python implementation

• Ease of understanding for other learning about how scrypt works
• Environments which do not allow modules

I have not had time to document performance, but it will not be good. :)