gitweb.css: constrain vertical height of readme
Add the necessary style elements so that any overly long readme
contents ends up inside its own scrollable element.
This prevents pushing the "standard" contents that follow the readme
(shortlog, tags, heads and forks) so far off the screen that it
takes forever to scroll there.
While not a "perfect solution," the result should be an improvement
when encountering a readme that renders as hundreds of lines.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>