Docs - add documentation for `unflatten()` - fix a typo in docstring
0.0.3
New - implement `unflatten` (2)
0.0.2
Fix - accept `collections.Mapping` as input (oringinally `collections.MutableMapping`) (1c6d5d41c39a7cbbe895b8c6d1f18b709f0e6773) - correctly raise error when invert with duplicate value (c7d8697cc5b00ebaa6ee12bf2d9edbbd1718c836)