Added markdown syntax cheat sheet link #221

This is my first stab at adding a markdown syntax cheat sheet link to
help people compose their posts (issue #221). The link opens a github
hosted markdown guide in a new tab.
@davidbalbert I put it into thread.css.scss because that's where the rules for the new post form (ie `div.new-post`) seem to be written. I didn't see any style entries for `div.new-post` in post.css.scss so I followed convention. Is that right?