Adding README for AUXIO

This pull request adds the "master" level README for the AUXIO system, and describes in general the process for setting up the AUXIO system, and how to use multiple IO systems at once.
recommend rewording - "Note: it is not necessary to pass these library flags to chpl compiler invocations as the values in CHPL_AUXIO_LIBS and CHPL_AUXIO_INCLUDE will be used there as well as in building the Chapel runtime"