v0.1.0 2018/05/01 - build 11

Included in both TeXLive and MiKTeX.

tlmgr install includeRnw

Download includeRnw.zip:

\documentclass{article}
\input{includeRnw}
\makeatletter
\begin{document}
\includeRnw{/path/to/file.Rnw}
\end{document}

You can send the h-option to prevent it from building the Rnw, and only including the already knitted file:

\includeRnw[h]{/path/to/file.Rnw}