Mark release 5.6.0

* Fix bug where you could paste into a `readOnly` editor
* Show a cursor at the drop location when dragging over the editor
* The [Rust mode](http://codemirror.net/mode/rust/index.html) was
  rewritten to handle modern Rust
* The editor and theme CSS was cleaned up. Some selectors are now
  less specific than before
* New theme: [abcdef](http://codemirror.net/demo/theme.html#abcdef)
* Lines longer than [`maxHighlightLength`](http://codemirror.net/doc/manual.html#option_maxHighlightLength)
  are now less likely to mess up indentation
* New addons: [`autorefresh`](http://codemirror.net/doc/manual.html#addon_autorefresh)
  for refreshing an editor the first time it becomes visible, and `html-lint` for using [HTMLHint](http://htmlhint.com/)
* The [`search`](http://codemirror.net/doc/manual.html#addon_search)
  addon now recognizes `\r` and `\n` in pattern and replacement input