readme typos

Thanks for merging in those things. Will try to think up a better way of giving leaves (it worked for what i needed at the time :) )

Two minor typo's in the readme. It should be:

# Remove a node link its children to its parent (root is not allowed)
t.link_past_node(nid)

instead of:

# Remove a node link its children to its parent (root is not allowd)
r.link_past_node(nid)