[Rd] Experimental CI tool for R

[Jeroen Ooms]

Based on ideas from the R-core discussion panel at useR2020, I created
a little CI tool to make it easier to follow changes in R-devel, and
to write/test patches for R.

The tool is based on a Github mirror of the SVN, where each new commit
triggers a full make-check on 8 different system configurations. The
results are published on: https://r-devel.github.io which gives an
overview of the most recent revisions, including links to the build
logs, and a link to the (unsigned) Windows installer. As of yesterday,
it should be possible to inspect the build logs without signing in to
GitHub.

The system can also be used to develop and test patches for base-R.
Anyone can send pull-requests, which will trigger the same set of
builds. The check results and link to Windows installer will appear
under your pull request. Finally, GitHub makes it very easy to export
a pull request as a patch file, which is the format that R-core
members still like to use. More instructions are available on:
https://github.com/r-devel/r-svn#readme

I hope this tool can make cross-platform testing and contributing of
base-R slightly less painful, while we are still on SVN.