Document QuickCheck

[mleonhard]

The (Quickstart Guide)[http://www.functionaljava.org/quickstart.html] says to add org.functionaljava:functionaljava-quickcheck:4.8.1 to my project. I read a bit about the QuickCheck technique. It looks very powerful and useful. I want to learn how it works, try it out, potentially use it to improve testing of my company's code.

I found no usable documentation on this module. The (functionaljava-quickcheck javadoc)[http://www.functionaljava.org/javadoc/4.8.1/functionaljava-quickcheck/index.html] has no overview information or examples.

Please add documentation of FJ's QuickCheck. Add a tutorial and examples for each main kind of test.

[gliptak]

@mleonhard please consider contributing as Javadoc or to the site at https://github.com/functionaljava/functionaljava.github.io/tree/gradle/src/jbake/content