HPCC-11743 Docs R Support instructions

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
Merge pull request #6147 from JamesDeFabia/11743RInsideInstallInstructions

HPCC-11743 Docs R Support instructions

Reviewed-By: Xiaoming Wang <xiaoming.wang@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>