Add introduction.

docs/index.rst

@@ -6,6 +6,7 @@ Contents:
 .. toctree::
    :maxdepth: 2
 
+   introduction
    instances
    typealiases
    references

docs/introduction.rst

+************
+Introduction
+************
+
+Yangson is a Python library that offers programmers tools for working with
+configuration and other data modelled with YANG [Bjo16]_. It is not
+intended to aid YANG module development, pyang__ is better suited for
+this purpose.
+
+Yangson supports only the JSON data encoding as defined in [Lho16]_.  
+
+__ https://github.com/mbj4668/pyang
+
+Yangson Modules
+***************
+
+The Yangson package comprises the following Python modules:
+
+:mod:`datamodel`
+    This module defines the *DataModel* class that is designed as the
+    main entry point to the library. It allows for composing a data
+    model based on YANG library [BBW16]_.
+
+:mod:`schema`
+    This module defines a class hierarchy representing YANG schema
+    nodes and their properties.
+
+:mod:`datatype`
+    This module defines a class hierarchy representing YANG built-in
+    and derived types.
+
+:mod:`instance`
+    This module defines several classes that implement data instances
+    as `persistent structures`_.
+
+:mod:`statement`
+    This module defines the :class:`Statement` class tha
+
+:mod:`modparser`
+    This module implements a parser for YANG modules and submodules.
+
+:mod:`context`
+    This module defines the *Context* class that represents the
+    context of the current YANG data model – YANG modules and
+    submodules with their revision, namespaces of the (sub)modules,
+    and the mapping of namespace prefixes to modules and
+    revisions. This context is shared and accessed by several other
+    Yangson modules. Only one instance of the *Context* class may
+    exist at the same time.
+
+:mod:`typealiases`
+    This module defines a number of Python type aliases that can be
+    used in type hints [PEP484]_.
+
+:mod:`constants`
+    This module defines various constants that are used in other
+    modules: base exception class, compiled regular expressions, and
+    enumeration classes. 
+
+The modules :mod:`typealiases` and :mod:`constants` are imported by
+all other modules. Other import dependencies are indicated in the
+following diagram.
+
+.. image:: imports.png
+
+.. _persistent structures: https://en.wikipedia.org/wiki/Persistent_data_structure
+