Create README.md

Author: andypbarrett

README.md

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+# xarray_open_tutorial
+
+Notebook and data for short xarray open tutorial presented at ICESat-2 Hackweek, Seattle, WA, 17 to 21 June, 2019.
+
+The notebook contains a **very** short introduction to the python xarray package.  Please see 
+http://xarray.pydata.org/en/stable/ for a more in depth introduction and documentation.
+
+I've included an example of **vectorized indexing**, which, for example, could be used to extract values from gridded data
+along an ICESat-2 track.  I've also included an example of how to interpolate an xarray field to a set of points.  The 
+`segtrax` team use this to interpolate ice motion vectors to the locations of ICESat-2 sea ice freeboard segments.  The 
+interpolation routine is based on one presented by Johan and Fernando Paolo.