update readme

Author: Joe Hamman

README.md

@@ -1,4 +1,4 @@
-# xray-data
+# xarray-data
 
-This repository exists to hold example datasets for xray (e.g., as used in the
-documentation) that would bloat the main repository if included there.
+This repository exists to hold example datasets for [xarray](http://xarray.pydata.org) (e.g., as used in the
+documentation) that would bloat the [main repository](https://github.com/pydata/xarray) if included there.