Edit README.md

README.md:
    Fixed whitespace.

    Made more succinct.
Make Julien's review suggestions

README.md:
    Remove references to lcc (hasn't been tried with Mercury for years)
    and the old .NET backend from README.md (likely to cause confusion
    more than anything else).

README.md:
README.IRIX-5:
    Remove the references to IRIX-5 (including the file README.IRIX-5)
    completely.  IRIX is pretty much dead, this documentation is pretty
    much clutter at this point.

README.AIX:
README.HPUX:
    Replacing the contents of README.HPUX and README.AIX to say that
    Mercury (to the best of our knowledge) hasn't been tried with recent
    versions of those OSs.