Add badges and a short description to the readme. (#13)

CHANGES.rst

@@ -15,6 +15,7 @@ releases are available on `Anaconda.org
   :func:`pybaum.tree_util.tree_just_yield`.
 - :gh:`10` adds default arguments to ``tree_just_flatten``.
 - :gh:`12` adds a section about the API to the docs.
+- :gh:`13` extends the readme.
 
 
 0.1.0 - 2022-01-28

README.rst

@@ -1,4 +1,91 @@
 pybaum
 ======
 
-Tools to work with pytrees.
+.. start-badges
+
+.. image:: https://img.shields.io/pypi/v/pybaum?color=blue
+    :alt: PyPI
+    :target: https://pypi.org/project/pybaum
+
+.. image:: https://img.shields.io/pypi/pyversions/pybaum
+    :alt: PyPI - Python Version
+    :target: https://pypi.org/project/pybaum
+
+.. image:: https://img.shields.io/conda/vn/conda-forge/pybaum.svg
+    :target: https://anaconda.org/conda-forge/pybaum
+
+.. image:: https://img.shields.io/conda/pn/conda-forge/pybaum.svg
+    :target: https://anaconda.org/conda-forge/pybaum
+
+.. image:: https://img.shields.io/pypi/l/pybaum
+    :alt: PyPI - License
+    :target: https://pypi.org/project/pybaum
+
+.. image:: https://readthedocs.org/projects/pybaum/badge/?version=latest
+    :target: https://pybaum.readthedocs.io/en/latest
+
+.. image:: https://img.shields.io/github/workflow/status/OpenSourceEconomics/pybaum/main/main
+   :target: https://github.com/OpenSourceEconomics/pybaum/actions?query=branch%3Amain
+
+.. image:: https://codecov.io/gh/OpenSourceEconomics/pybaum/branch/main/graph/badge.svg
+    :target: https://codecov.io/gh/OpenSourceEconomics/pybaum
+
+.. image:: https://results.pre-commit.ci/badge/github/OpenSourceEconomics/pybaum/main.svg
+    :target: https://results.pre-commit.ci/latest/github/OpenSourceEconomics/pybaum/main
+    :alt: pre-commit.ci status
+
+.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
+    :target: https://github.com/psf/black
+
+.. end-badges
+
+Installation
+------------
+
+pybaum is available on `PyPI <https://pypi.org/project/pybaum>`_ and `Anaconda.org
+<https://anaconda.org/conda-forge/pybaum>`_. Install it with
+
+.. code-block:: console
+
+    $ pip install pybaum
+
+    # or
+
+    $ conda install -c conda-forge pybaum
+
+
+About
+-----
+
+pybaum provides tools to work with pytrees which is a concept burrowed from `JAX
+<https://jax.readthedocs.io/en/latest/>`_.
+
+What is a pytree?
+
+In pybaum, we use the term pytree to refer to a tree-like structure built out of
+container-like Python objects. Classes are considered container-like if they are in the
+pytree registry, which by default includes lists, tuples, and dicts. That is:
+
+1. Any object whose type is not in the pytree container registry is considered a leaf
+   pytree.
+
+2. Any object whose type is in the pytree container registry, and which contains
+   pytrees, is considered a pytree.
+
+For each entry in the pytree container registry, a container-like type is registered
+with a pair of functions that specify how to convert an instance of the container type
+to a (children, metadata) pair and how to convert such a pair back to an instance of the
+container type. Using these functions, pybaum can canonicalize any tree of registered
+container objects into tuples.
+
+Example pytrees:
+
+.. code-block:: python
+
+    [1, "a", object()]  # 3 leaves
+
+    (1, (2, 3), ())  # 3 leaves
+
+    [1, {"k1": 2, "k2": (3, 4)}, 5]  # 5 leaves
+
+pybaum can be extended to consider other container types as pytrees.