README: add stability badge
-# Gonum [![Build Status](https://www.travis-ci.org/gonum/gonum.svg?branch=master)](https://www.travis-ci.org/gonum/gonum/branches) [![Build status](https://ci.appveyor.com/api/projects/status/valslkp8sr50eepn/branch/master?svg=true)](https://ci.appveyor.com/project/Gonum/gonum/branch/master) [![Coverage Status](https://coveralls.io/repos/gonum/gonum/badge.svg?branch=master&service=github)](https://coveralls.io/github/gonum/gonum?branch=master) [![GoDoc](https://godoc.org/gonum.org/v1/gonum?status.svg)](https://godoc.org/gonum.org/v1/gonum) [![Go Report Card](https://goreportcard.com/badge/github.com/gonum/gonum)](https://goreportcard.com/report/github.com/gonum/gonum)
+# Gonum [![Build Status](https://www.travis-ci.org/gonum/gonum.svg?branch=master)](https://www.travis-ci.org/gonum/gonum/branches) [![Build status](https://ci.appveyor.com/api/projects/status/valslkp8sr50eepn/branch/master?svg=true)](https://ci.appveyor.com/project/Gonum/gonum/branch/master) [![Coverage Status](https://coveralls.io/repos/gonum/gonum/badge.svg?branch=master&service=github)](https://coveralls.io/github/gonum/gonum?branch=master) [![GoDoc](https://godoc.org/gonum.org/v1/gonum?status.svg)](https://godoc.org/gonum.org/v1/gonum) [![Go Report Card](https://goreportcard.com/badge/github.com/gonum/gonum)](https://goreportcard.com/report/github.com/gonum/gonum) [![stability-unstable](https://img.shields.io/badge/stability-unstable-yellow.svg)](https://github.com/emersion/stability-badges#unstable)
## Installation
