Add brief overview to readme (#152)

* add readme

* add examples

* review actions

* provide links to example files

Author: stephenworsley

README.md

@@ -7,3 +7,59 @@
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+
+---
+
+## Overview
+
+This project aims to provide a bridge between [Iris](https://github.com/SciTools/iris)
+and [ESMF](https://github.com/esmf-org/esmf). This takes the form of regridder classes
+which take Iris cubes as their arguments and use ESMF to perform regridding
+calculations. These classes are designed to perform well on cubes which have multiple
+non-horizontal dimensions and lazy ([Dask](https://github.com/dask/dask)) data.
+Both rectilinear and curvilinear grids as well as UGRID meshes have been supported.
+
+## Regridding Example
+
+There are a range of regridder classes (e.g `MeshToGridESMFRegridder` and
+`GridToMeshESMFRegridder`). For an example of the regridding process, the
+`MeshToGridESMFRegridder` class works as follows:
+
+```python
+import iris
+from iris.experimental.ugrid import PARSE_UGRID_ON_LOAD
+from esmf_regrid.experimental.unstructured_scheme import MeshToGridESMFRegridder
+
+# An example such a file can be found at:
+# https://github.com/SciTools/iris-test-data/blob/master/test_data/NetCDF/unstructured_grid/data_C4.nc
+with PARSE_UGRID_ON_LOAD.context():
+    source_mesh_cube = iris.load_cube("mesh_cube.nc")
+
+# An example of such a file can be found at:
+# https://github.com/SciTools/iris-test-data/blob/master/test_data/NetCDF/global/xyt/SMALL_hires_wind_u_for_ipcc4.nc
+target_grid_cube = iris.load_cube("grid_cube.nc")
+
+# Initialise the regridder with a source mesh and target grid.
+regridder = MeshToGridESMFRegridder(source_mesh_cube, target_grid_cube)
+
+# use the initialised regridder to regrid the data from the source cube
+# onto a cube with the same grid as `target_grid_cube`.
+result = regridder(source_mesh_cube)
+```
+
+Note that this pattern allows the reuse of an initialised regridder, saving
+significant amounts of time when regridding. To make use of this efficiency across
+sessions, we support the saving of certain regridders. We can do this as follows:
+
+```python
+from esmf_regrid.experimental.io import load_regridder, save_regridder
+
+# Save the regridder.
+save_regridder(regridder, "saved_regridder.nc")
+
+# Load saved regridder.
+loaded_regridder = load_regridder("saved_regridder.nc")
+
+# Use loaded regridder.
+result = loaded_regridder(source_mesh_cube)
+```