Better readme

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 immutables
 ==========
 
+.. image:: https://travis-ci.org/MagicStack/immutables.svg?branch=master
+    :target: https://travis-ci.org/MagicStack/immutables
+
+.. image:: https://ci.appveyor.com/api/projects/status/tgbc6tq56u63qqhf?svg=true
+    :target: https://ci.appveyor.com/project/MagicStack/immutables
+
 An immutable mapping type for Python.
 
 The underlying datastructure is a Hash Array Mapped Trie (HAMT)
-used in Clojure and other functional languages.  The actual
-implementation is used in CPython 3.7 (see PEP 567 and
-the contextvars module.)
+used in Clojure, Scala, Haskell, and other functional languages.
+This implementation is used in CPython 3.7 in the ``contextvars``
+module (see PEP 550 and PEP 567 for more details).
+
+Immutable mappings based on HAMT have O(log\ :sub:`32`\ N)
+performance for both ``set()`` and ``get()`` operations, which is
+essentially O(1) for relatively small mappings.
+
+Below is a visualization of a simple get/set benchmark comparing
+HAMT to an immutable mapping implemented with a Python dict
+copy-on-write approach (the benchmark code is available
+`here <https://gist.github.com/1st1/9004813d5576c96529527d44c5457dcd>`_):
+
+.. image:: bench.png
+
+
+Installation
+------------
+
+``immutables`` requires Python 3.5+ and is available on PyPI::
+
+    $ pip install immutables
+
+
+immutables.Map
+--------------
+
+The ``Map`` object implements ``collections.abc.Mapping`` ABC
+so working with it is very similar to working with Python dicts.
+
+The only exception is its ``Map.set()`` and ``Map.delete()`` methods
+which return a new instance of ``Map``:
+
+.. code-block:: python
+
+    m1 = Map()  # an empty Map
+    m2 = m1.set('key1', 'val1')  # m2 has a 'key1' key, m1 is still empty
+
+    m3 = m2.set('key2', 'val2')
+    m3 = m3.delete('key1')  # m3 has only a 'key2' key
+
+
+Further development
+-------------------
+
+* An immutable version of Python ``set`` type with efficient
+  ``add()`` and ``discard()`` operations.
+
+* Add support for efficient ``Map.update()`` operation, allow to
+  pass a set of key/values to ``Map()``, and add support for
+  pickling.
 
 
 License
-=======
+-------
 
 Apache 2.0