mention the dpll solver in the readme, document usage

rust/solve/README.md

That being said, this is me playing around with Rust and writing some
code related to constraint programming at the same time.
Currently there's an incomplete parser for CNFs in DIMAC format in
[./src/cnf.rs](./src/cnf.rs).
There are a few things here:
- an incomplete parser for CNFs in DIMAC format ([./src/cnf.rs](./src/cnf.rs))
- a naïve implementation of the DPLL algorithm
- a binary that uses these things to parse CNFs on stdin and solves them
    using the DPLL implementation
    you can try it yourself:
        $ cat examples/external-02-quinn.cnf | cargo run --release