add stable docs badge [ci skip]

Author: bicycle1885

README.md

@@ -2,6 +2,7 @@ TranscodingStreams.jl
 =====================
 
 <!--[![Appveyor Status][appveyor-img]][appveyor-url]-->
+[![Docs Stable][docs-stable-img]][docs-stable-url]
 [![Docs Latest][docs-latest-img]][docs-latest-url]
 [![TravisCI Status][travisci-img]][travisci-url]
 [![codecov.io][codecov-img]][codecov-url]
@@ -20,5 +21,7 @@ Consult the [docs][docs-latest-url] for more details and examples.
 [travisci-url]: https://travis-ci.org/bicycle1885/TranscodingStreams.jl
 [codecov-img]: http://codecov.io/github/bicycle1885/TranscodingStreams.jl/coverage.svg?branch=master
 [codecov-url]: http://codecov.io/github/bicycle1885/TranscodingStreams.jl?branch=master
+[docs-stable-img]: https://img.shields.io/badge/docs-stable-blue.svg
+[docs-stable-url]: https://bicycle1885.github.io/TranscodingStreams.jl/stable/
 [docs-latest-img]: https://img.shields.io/badge/docs-latest-blue.svg
 [docs-latest-url]: https://bicycle1885.github.io/TranscodingStreams.jl/latest/