RUST-730 Add MSRV policy to documentation (#444)

Author: patrickfreed

README.md

@@ -351,6 +351,11 @@ bash .evergreen/check-all.sh
 ## Continuous Integration
 Commits to master are run automatically on [evergreen](https://evergreen.mongodb.com/waterfall/mongo-rust-driver-stable).
 
+## Minimum supported Rust version (MSRV)
+
+The MSRV for this crate is currently 1.48.0. This will be rarely be increased, and if it ever is,
+it will only happen in a minor or major version release.
+
 ## License
 
 This project is licensed under the [Apache License 2.0](https://github.com/10gen/mongo-rust-driver/blob/master/LICENSE).

src/lib.rs

@@ -275,6 +275,11 @@
 //! # Ok(())
 //! # }
 //! ```
+//!
+//! ## Minimum supported Rust version (MSRV)
+//!
+//! The MSRV for this crate is currently 1.48.0. This will be rarely be increased, and if it ever is,
+//! it will only happen in a minor or major version release.
 
 #![warn(missing_docs)]
 #![warn(missing_crate_level_docs)]