Add DMA to top-level docs

Author: richardeoin

src/lib.rs

@@ -43,6 +43,7 @@
 //!
 //! Others
 //!
+//! * [Direct Memory Access (DMA)](crate::dma)
 //! * [Cyclic Redundancy Check (CRC)](crate::crc) Feature gate `crc`
 //! * [Random Number Generator](crate::rng) ([rand_core::RngCore](rand_core::RngCore) is implemented under the`rand` feature gate)
 //! * [System Window Watchdog](crate::watchdog)