Add a section to the documentation on differences with previous versions of ``astrodendro``

Examples:

* ``f`` -> ``values``
* ``fmin/fmax`` -> ``vmin/vmax``
* ``coords`` -> ``indices``
* definition of ``height`` has changed
* ``merge_level`` has been removed

Would be useful to have a dedicated page in the docs, just for anyone who's previously used the old developer version.