Removing content from README.md

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.