Removing content from README.md

[Robert Goldman]

As someone who doesn't use the gitlab interface, it was only now that I
realized that there was a README.md.

I just looked over that README, and realized that it was not maintained.
I quickly concluded that I do not have the resources to maintain both
this file and the HTML file at https://common-lisp.net/project/asdf/,
and the latter is what I have been updating with new releases.  So the
former must go.

I apologize for this draconian step.

Of course nothing has been lost, it is all in the git history.

I am willing to entertain the argument that the index.html be deprecated
instead of the Markdown page.  However, I will not entertain requests
for both.  One will have to be merely a pointer to the other.