README: explicitly state version support policy

.travis.yml

@@ -2,6 +2,7 @@ sudo: false
 
 language: go
 
+# Do not move these lines; they are referred to by README.md.
 # Versions of go that are explicitly supported by gonum plus go tip.
 go:
  - 1.8.x

README.md

@@ -8,6 +8,11 @@ Installation is done using `go get`.
 go get -u gonum.org/v1/gonum/...
 ```
 
+## Supported Go versions
+
+Gonum supports and tests on the [three most recent major versions of Go](https://github.com/gonum/gonum/blob/master/.travis.yml#L6-L11).
+
+
 ## Build tags
 
 The gonum packages use a variety of build tags to set non-standard build conditions.