add link to github.io site from README.md

README.md

@@ -4,3 +4,5 @@ Rscons (https://github.com/holtrop/rscons) is an open-source build system
 for developers.
 
 Rscons is written in Ruby, and is inspired by [SCons](https://scons.org/) and [waf](https://waf.io/).
+
+See [https://holtrop.github.io/rscons/index.html](https://holtrop.github.io/rscons/index.html) for User Guide and Installation instructions.