docs: Add quick docs for uzlib.

docs/library/index.rst

@@ -44,6 +44,7 @@ it will fallback to loading the built-in ``ujson`` module.
    ujson.rst
    ure.rst
    usocket.rst
+   uzlib.rst
 
 Libraries specific to the pyboard
 ---------------------------------

docs/library/uzlib.rst

@@ -0,0 +1,16 @@
+:mod:`uzlib` -- zlib decompression
+==================================
+
+.. module:: uzlib
+   :synopsis: zlib decompression
+
+This modules allows to decompress binary data compressed with DEFLATE
+algorithm (commonly used in zlib libarary and gzip archiver). Compression
+is not yet implemented.
+
+Functions
+---------
+
+.. function:: decompress(data)
+
+   Return decompressed data as bytes.