README: Move to markdown

I'm working on the cangjians.github.io website, and I'm using markdown
for it.

Moving this file to markdown allows me to reuse it as a page in the
website, without nay modification, which is quite nice.
README: Point to the online documentation
README: Drop the mention to the INSTALL.rst page

It's so obvious that the installation instructions are in the file named
INSTALL...
README: Drop the mention to the AUTHORS file

People are telling me that they don't find the AUTHORS file in the Git
tree, which is normal as it's generated when creating the tarball. I can
understand it is confusing, though.

It's also becoming problematic when we reuse the README as a page of the
website.

All in all, let's remove it and avoid the confusion.