Add content to Index page xarray-contrib/datatree#182

* Joe's suggestions for index page

* whatsnew

Author: TomNicholas

docs/source/index.rst

@@ -3,6 +3,38 @@ Datatree
 
 **Datatree is a prototype implementation of a tree-like hierarchical data structure for xarray.**
 
+Why Datatree?
+~~~~~~~~~~~~~
+
+Datatree was born after the xarray team recognised a `need for a new hierarchical data structure <https://github.com/pydata/xarray/issues/4118>`_,
+that was more flexible than a single :py:class:`xarray.Dataset` object.
+The initial motivation was to represent netCDF files / Zarr stores with multiple nested groups in a single in-memory object,
+but :py:class:`~datatree.DataTree` objects have many other uses.
+
+You might want to use datatree for:
+
+- Organising many related datasets, e.g. results of the same experiment with different parameters, or simulations of the same system using different models,
+- Analysing similar data at multiple resolutions simultaneously, such as when doing a convergence study,
+- Comparing heterogenous but related data, such as experimental and theoretical data,
+- I/O with nested data formats such as netCDF / Zarr groups.
+
+Development Roadmap
+~~~~~~~~~~~~~~~~~~~
+
+Datatree currently lives in a separate repository to the main xarray package.
+This allows the datatree developers to make changes to it, experiment, and improve it faster.
+
+Eventually we plan to fully integrate datatree upstream into xarray's main codebase, at which point the `github.com/xarray-contrib/datatree <https://github.com/xarray-contrib/datatree>`_ repository will be archived.
+This should not cause much disruption to code that depends on datatree - you will likely only have to change the import line (i.e. from ``from datatree import DataTree`` to ``from xarray import DataTree``).
+
+However, until this full integration occurs, datatree's API should not be considered to have the same `level of stability as xarray's <https://docs.xarray.dev/en/stable/contributing.html#backwards-compatibility>`_.
+
+User Feedback
+~~~~~~~~~~~~~
+
+We really really really want to hear your opinions on datatree!
+At this point in development, user feedback is critical to help us create something that will suit everyone's needs.
+Please raise any thoughts, issues, suggestions or bugs, no matter how small or large, on the `github issue tracker <https://github.com/xarray-contrib/datatree/issues>`_.
 
 .. toctree::
    :maxdepth: 2
@@ -18,7 +50,6 @@ Datatree
    Terminology <terminology>
    How do I ... <howdoi>
    Contributing Guide <contributing>
-   Development Roadmap <roadmap>
    What's New <whats-new>
    GitHub repository <https://github.com/xarray-contrib/datatree>
 

docs/source/whats-new.rst

@@ -60,6 +60,8 @@ Documentation
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
 - Added page on ``Working with Hierarchical Data`` (:pull:`179`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
+- Added context content to ``Index`` page (:pull:`182`)
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Internal Changes
 ~~~~~~~~~~~~~~~~