Added sf-tree-history to README

Author: simonw

README.md

@@ -3,7 +3,7 @@
 [![PyPI](https://img.shields.io/pypi/v/csv-diff.svg)](https://pypi.org/project/csv-diff/)
 [![License](https://img.shields.io/badge/license-Apache%202.0-blue.svg)](https://github.com/simonw/csv-diff/blob/master/LICENSE)
 
-Tool for viewing the difference between two CSV files.
+Tool for viewing the difference between two CSV files. See [Generating a commit log for San Francisco’s official list of trees](https://simonwillison.net/2019/Mar/13/tree-history/) (and the [sf-tree-history repo commit log](https://github.com/simonw/sf-tree-history/commits)) for background information on this project.
 
 Consider two CSV files:
 
@@ -82,4 +82,4 @@ You can also import the Python library into your own code like so:
         load_csv(open("two.csv"), key="id")
     )
 
-`diff` will now contain the same data structure as the output in the `--json` example above.
+`diff` will now contain the same data structure as the output in the `--json` example above.