add a tiny bit of docs

rust/dpll/src/cnf.rs

//! The cnf library provides facilities to work with CNF formulas.
//!
//! `CNF` stands for conjunctive normal form, i.e. a logic formula that
//! consists of a conjunction of disjunctions.
 pub struct CNF {
	  pub num_vars: u32,
	  pub num_clauses: u32,

rust/dpll/src/lib.rs

//! This is a tiny library for building a SAT solver in Rust.
pub mod cnf;