Add documentation

[watzon]

This seems to be a great library and it has everything I need, but it's borderline unusable because of the lack of documentation. It would be great if people that understand what's going on could go through and document all of the code, add examples, and make the experience a little better.

[AKushWarrior]

I wrote a wrapper library for PointyCastle to solve this problem, and I'm in the process of porting all the API's. If you want, take a look at https://pub.dev/packages/steel_crypt#-readme-tab-