Added <br/> and <seealso cref=xxx>

[mikelau13]

[cpboyd]

I've created my own fork and taken an initial stab at trying to fix anchor links and such: https://github.com/cpboyd/XmlCommentMarkDownGenerator/commit/fc0fab724c0e8b730242e1673e09e478a9df517b

It also adds other things beyond the scope of this PR, like a few more HTML and Sandcastle/NDoc tags.