Create README.md

(Commit log by Paul Bone)

Github will automatically format and display a file named README.md in the
root directory of any project.  This can be helpful to, at a minimum, direct
anyone browsing github to the project's website.  Sebastian has contributed
the following README.md.

README.md:
    New markdown-formatted README file for github.
README.md changes

README.md:
    Changed intro to www.mercurylang.org introduction.

    Added a NOTES and LIMITATIONS section.

    Added a link to the README.bootstrap.
Make a number of changes to the README.md file

README.md:
    Reorder the list or README files to that they occur from most relevant
    to new users to least relevant.  In particular put commonly-used
    backends and platforms earlier in the list.

    Provide some introductory text that introduce the user to the multiple
    backends available.

    Add further links to more information such as the documentation.

    Add text so that the file reads more natrually.
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.
Merge branch 'readme'

Merge in the changes that introduce and fxiup the README.md file.