Update README for unref/ref APIs.
Update README for unref/ref APIs.

 * Also fix some Markdown incompatibilities with 3rd party editors.

hhamilto/zeromq.node,JustinTulloss/zeromq.node,lablup/zeromq.node,JustinTulloss/zeromq.node,JustinTulloss/zeromq.node,lablup/zeromq.node,hhamilto/zeromq.node,hhamilto/zeromq.node,lablup/zeromq.node,lablup/zeromq.node,hhamilto/zeromq.node,JustinTulloss/zeromq.node