docs: Start introduction.

pywr-book/src/introduction.md

@@ -1,3 +1,15 @@
 # Introduction
 
-Welcome to the Pywr book! Pywr is an open-source network resource allocation model.
+Welcome to the Pywr book!
+Pywr is an open-source network resource allocation model.
+This book is a collection of documentation and tutorials for using Pywr.
+It should be read alongside the Pywr API documentation.
+
+# What is Pywr?
+
+Pywr is a Rust crate and Python package for building and running water resource models.
+It allows users to construct models of water systems using a network of nodes and links, and other data.
+The model can then be used to simulate the operation of a water system, and to evaluate the performance of the system under different scenarios.
+
+This version is a major update to the original Pywr model, which was written in Python and Cython.
+The new version is written in Rust, and uses Python bindings to expose the functionality to Python.