add readme based on module docstring

Author: ivilata

README.txt

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+======================================================
+ Python implementation of the multihash specification
+======================================================
+
+This is an implementation of the `multihash`_ specification in Python.
+The main component in the module is the `Multihash` class, a named tuple that
+represents a hash function and a digest created with it, with extended
+abilities to work with hashlib-compatible hash functions, verify the integrity
+of data, and encode itself to a byte string in the binary format described in
+the specification.  The `decode()` function can be used for the inverse
+operation, i.e. converting a byte string into a `Multihash` object.
+
+.. _multihash: https://github.com/jbenet/multihash
+
+Please see the docstring in the ``multihash`` module for more information.