Add MSRV policy

.travis.yml

@@ -7,6 +7,7 @@ env:
 rust:
   - nightly
   - stable
+  - 1.30.0 # MSRV
 
 if: (branch = staging OR branch = trying OR branch = master) OR (type = pull_request AND branch = master)
 

src/lib.rs

@@ -1,5 +1,14 @@
 //! Low level access to RISC-V processors
 //!
+//! # Minimum Supported Rust Version (MSRV)
+//!
+//! This crate is guaranteed to compile on stable Rust 1.30 and up. It *might*
+//! compile with older versions but that may change in any new patch release.
+//! Note that `riscv64imac-unknown-none-elf` and `riscv64gc-unknown-none-elf` targets
+//! are not supported on stable yet.
+//!
+//! # Features
+//!
 //! This crate provides:
 //!
 //! - Access to core registers like `mstatus` or `mcause`.