README: add "docs passing" badge

Author: litespeedtech

README.md

@@ -1,6 +1,7 @@
 [![Linux and MacOS build status](https://ci.appveyor.com/api/projects/status/x790ve5msewmva2b/branch/master?svg=true)](https://ci.appveyor.com/project/litespeedtech/lsquic-linux/branch/master)
 [![Windows build status](https://ci.appveyor.com/api/projects/status/ij4n3vy343pkgm1j/branch/master?svg=true)](https://ci.appveyor.com/project/litespeedtech/lsquic-windows/branch/master)
 [![FreeBSD build status](https://api.cirrus-ci.com/github/litespeedtech/lsquic.svg)](https://cirrus-ci.com/github/litespeedtech/lsquic)
+[![Documentation Status](https://readthedocs.org/projects/lsquic/badge/?version=latest)](https://lsquic.readthedocs.io/en/latest/?badge=latest)
 
 LiteSpeed QUIC (LSQUIC) Library README
 =============================================