Remove the warning in README.md about versioning.

Now that v1.0.0 is being released and will use a proper versioning mechanism,
there's no need for this warning.

Clients will be able to use http://github.com/golang/dep to version this
library.

README.md

@@ -11,15 +11,6 @@ Documentation is at https://periph.io
 [![Gitter chat](https://badges.gitter.im/google/periph.png)](https://gitter.im/periph-io/Lobby)
 
 
-## WARNING
-
-**This library is unstable. Breaking API changes occur frequently.**
-
-If you want to use `periph.io/x/periph`, please [vendor
-periph](https://github.com/golang/go/wiki/PackageManagementTools) first and
-frequently roll.
-
-
 ## Example
 
 ~~~go