Proper comparison to other libraries with code examples

[bruderj15]

Current README only has a few senctences on related work.

It does not quite highlight, what Hasmtlib does better than sbv, simple-smt, what4, smtlib2.

Explicitly show what we do better and provide small code comparisons, mainly showing how good embedded the DSL is, whether formula construction is pure or monadic and how simple and safe the types are.
This may fit into the README.

However, comparison with a big example may also be worth it.
Maybe something like the encoding of knights tour or Sudoku?
This could also be used to compare performance of the libraries!