Update README

Changes/Additions
- :vsplit, :split, :unsplit, c-w to alternate between splits
- ñ mapped to : by default
- za, space mapped to za
- New entire document text-object, af (a file)
- Minimal, buggy implementation of the surround plugin
- Enter in normal mode inserts empty line below, stays in insert mode
- c-j is equivalent to 5j and c-k is equivalent to 5k
- Change commands and visual mode pasting don't overwrite the register