deno doc --html : incorporate the content of the readme.md file into the documentation (if available)

[iherman]

The issue title tells it all. It is always a good idea to write a GitHub readme.md file that would be some sort of an introduction to a package, but it should also be used as part of the documentation on the front page. Converting that file into the HTML index page is very helpful.

(I am obviously influenced by the fact that typedoc does exactly that, and I found that a really useful feature when writing a documentation.)