Add documentation

[atoponce]

The projects needs documentation discussing:

• How to build the project.
• Why bscrypt was created, such as comparison to other password hashing functions.
• How to implement bscrypt along with test vectors.
• Recommended cost settings and their different performance metrics.
• Why being cache hard is important and how it compares to memory hardness.
• Etc.

[Sc00bz]

I was working on this at DEFCON and on the plane home. Also most of this was planned but didn't have time. I should probably add a check list:

• How to build the project.
• Why bscrypt was created, such as comparison to other password hashing functions.
• How to implement bscrypt along with test vectors.
• Recommended cost settings and their different performance metrics.
• Why being cache hard is important and how it compares to memory hardness.
• AVX2
• More code documentation
• JS version

I'll probably commit changes tomorrow.