Bump minimum supported version & document support policy (#599)

* Bump minimum supported version & document support policy

Signed-off-by: Eliza Weisman <[email protected]>

.travis.yml

@@ -15,7 +15,7 @@ matrix:
   # This represents the minimum Rust version supported by Tokio. Updating this
   # should be done in a dedicated PR and cannot be greater than two 0.x
   # releases prior to the current stable.
-  - rust: 1.25.0
+  - rust: 1.26.0
   - rust: stable
   - rust: beta
   - rust: nightly

README.md

@@ -171,6 +171,14 @@ The crates included as part of Tokio are:
 
 This project is licensed under the [MIT license](LICENSE).
 
+## Supported Rust Versions
+
+Tokio is built against the latest stable, nightly, and beta Rust releases. The
+minimum version supported is the stable release from three months before the
+current stable release version. For example, if the latest stable Rust is 1.29,
+the minimum version supported is 1.26. The current Tokio version is not
+guaranteed to build on Rust versions earlier than the minimum supported version.
+
 ### Contribution
 
 Unless you explicitly state otherwise, any contribution intentionally submitted