Update PLDM README.md with information about pldm runtime module
Update PLDM README.md with information about pldm runtime module

Change-Id: If2320cc8fb3ebd9ae9f95f39b486fb4c83c4169c
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/97964
Reviewed-by: Roland Veloz <768fb49134ab823c765de70e7067d9f65f5d0a3e@us.ibm.com>
Tested-by: Jenkins Server <8e3f934e4c44875bc48d33da3ea13d93ba9a233f@us.ibm.com>
Tested-by: Jenkins OP Build CI <e610bd72261d3c0a48f1e8ae36832ab00774d426@us.ibm.com>
Tested-by: FSP CI Jenkins <aa9e4d9ac7cd25905e9c0dd36d4150516e73dd86@us.ibm.com>
Tested-by: Jenkins Combined Simics CI <8312af3886dfc9bb930f549023677220805b137e@us.ibm.com>
Reviewed-by: Matthew Raybuck <9bcecd56bb1097e934e4548e2b924569df303f40@ibm.com>
Reviewed-by: Daniel M Crowell <912029ca9254ac7b5854e56910561d682e1fa2d0@us.ibm.com>

open-power/hostboot,open-power/hostboot,open-power/hostboot,open-power/hostboot,open-power/hostboot