link the dpll source from the readme

rust/solve/README.md

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 naïve implementation of the DPLL algorithm ([./src/dpll.rs](./src/dpll.rs))
- a binary that uses these things to parse CNFs on stdin and solves them
    using the DPLL implementation