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.
Merge pull request #1 from PaulBone/sebgod-readme