Document MSRV with the defmt exception, don't test defmt with 1.40

.github/workflows/test.yml

@@ -70,7 +70,12 @@ jobs:
         features:
           # These feature sets cannot run tests, so we only check they build.
           - medium-ip medium-ethernet proto-ipv6 proto-ipv6 proto-igmp proto-dhcpv4 socket-raw socket-udp socket-tcp socket-icmp async
-          - defmt medium-ip medium-ethernet proto-ipv6 proto-ipv6 proto-igmp proto-dhcpv4 socket-raw socket-udp socket-tcp socket-icmp async
+        include:
+          # defmt doesn't support 1.40
+          - rust: stable
+            features: defmt medium-ip medium-ethernet proto-ipv6 proto-ipv6 proto-igmp proto-dhcpv4 socket-raw socket-udp socket-tcp socket-icmp async
+          - rust: nightly
+            features: defmt medium-ip medium-ethernet proto-ipv6 proto-ipv6 proto-igmp proto-dhcpv4 socket-raw socket-udp socket-tcp socket-icmp async
 
     steps:
       - uses: actions/checkout@v2

src/lib.rs

@@ -64,6 +64,14 @@
 //! feature ever defined, to ensure that, when the representation layer is unable to make sense
 //! of a packet, it is still logged correctly and in full.
 //!
+//! # Minimum Supported Rust Version (MSRV)
+//!
+//! This crate is guaranteed to compile on stable Rust 1.40 and up with any valid set of features.
+//! It *might* compile on older versions but that may change in any new patch release.
+//!
+//! The exception is when using the `defmt` feature, in which case `defmt`'s MSRV applies, which
+//! is higher than 1.40.
+//!
 //! [wire]: wire/index.html
 //! [osi]: https://en.wikipedia.org/wiki/OSI_model
 //! [berk]: https://en.wikipedia.org/wiki/Berkeley_sockets