Add README

README.rst

+**zlib_into** allows Python code to compress & decompress data into a
+preallocated buffer.
+
+The `zlib module <https://docs.python.org/3/library/zlib.html>`_ in the Python
+standard library provides an interface to zlib, a common library for Deflate
+compression, but always automatically allocates memory for the output.
+
+zlib_into defines two functions:
+
+.. code-block:: python
+
+    compress_into(data, output, level=-1, wbits=15)
+
+    decompress_into(data, output, wbits=15)
+
+In each case, ``output`` can be a ``bytearray``, ``memoryview``, Numpy array,
+or anything else compatible with Python's buffer protocol exposing a contiguous,
+writable chunk of memory.
+
+The other parameters have the same meanings as in `zlib.compress
+<https://docs.python.org/3/library/zlib.html#zlib.compress>`_ and
+`zlib.decompress <https://docs.python.org/3/library/zlib.html#zlib.decompress>`_.
+
+This can be useful for:
+
+- Decompressing regular chunks of a known size: you can allocate a single
+  output array and fill each chunk directly, avoiding an extra copy.
+- Compressing chunks when you want to skip compression if it would make the data
+  larger: use a fixed-size buffer matching the input chunk size. Again you can
+  avoid unnecessary memory allocations by reusing the buffer.
+- Imposing a size limit on decompressed data: it will stop decompressing (and
+  throw an error) when the buffer is full, instead of decompressing the whole
+  thing to see how big it is. But note that security on untrusted input is not
+  the primary goal of this library.