Add README.

xnd/README.rst

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+Overview
+--------
+
+The ``xnd`` module implements a container type that maps most Python values
+relevant for scientific computing directly to typed memory.
+
+Whenever possible, a single, pointer-free memory block is used.
+
+``xnd`` supports ragged arrays, categorical types, indexing, slicing, aligned
+memory blocks and type inference.
+
+Operations like indexing and slicing return zero-copy typed views on the data.
+
+Importing PEP-3118 buffers is supported.
+
+Links
+-----
+
+* https://github.com/xnd-project/