Describe versioning policy in README (#198)

.travis.yml

@@ -1,9 +1,8 @@
 sudo: false
 language: go
 go:
-  - 1.7.6
-  - 1.8.3
-  - 1.9
+  - 1.8.4
+  - 1.9.1
   - tip
 matrix:
   allow_failures:

README.md

@@ -114,6 +114,13 @@ You have to make sure two kind of tests run:
 
 You can run both of them using `./test.sh`.
 
+## Versioning
+
+Go-toml follows [Semantic Versioning](http://semver.org/). The supported version
+of [TOML](https://github.com/toml-lang/toml) is indicated at the beginning of
+this document. The last two major versions of Go are supported
+(see [Go Release Policy](https://golang.org/doc/devel/release.html#policy)).
+
 ## License
 
 The MIT License (MIT). Read [LICENSE](LICENSE).