Add HISTORY.md entry
# In progress
## Improvements
+- Added dataframe and CSV helper functions [#TDB](...)
+ `tiledb.{from_dataframe, from_csv, open_dataframe}`
- Added repr for ArraySchema [#267](https://github.com/TileDB-Inc/TileDB-Py/pull/267)
- Added repr for Attr [#266](https://github.com/TileDB-Inc/TileDB-Py/pull/266)
