Ports-related README updates

[Roland McGrath]

Looks fine to me.

> +git.  To report a bug in code housed in the ports add-on, please go to
> +http://sources.redhat.com/bugzilla/ and file a bug report under the
> +glibc "ports" component.

This is unrelated to your change and I just noticed it because you refilled
the paragraph, but all URLs should use sourceware.org nowadays rather than
sources.redhat.com.