Bump MSRV to 1.26.0 and Document Explicitly

Author: posborne

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 sudo: false
 rust:
-  - 1.22.1
+  - 1.26.0  # MSRV
   - stable
   - beta
   - nightly

README.md

@@ -74,6 +74,11 @@ The following features are implemented and planned for the library:
 - [x] Support for configuring spidev device
 - [ ] Support for querying spidev configuration state
 
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.26 and up.  It *might*
+compile with older versions but that may change in any new patch release.
+
 ## Cross Compiling
 
 Most likely, the machine you are running on is not your development