Add README for source code

Author: gjbex

source-code/README.md

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+#| Source code
+
+This is source code that is either used in the presentation, or was developed
+to create it.  There is some material not covered in the presentation as well.
+
+## Requirements
+
+* Python version: at least 3.6
+* Packages:
+  * pandas
+  * seaborn
+  * holoviews
+  * sqlalchemy
+  * folium
+  * matplotlib
+  * numpy
+  * scipy
+  * geopandas
+  * shapely
+  * bs4
+  * networkx
+  * Jupyter notebook
+* Optional packages
+  * gdal
+  * fiona
+
+## What is it?
+* [`db-access`](db-access): illustration of accessing SQLite databases and using
+  SQLAlchemy, including object-relational mapping.
+* [`gis`](gis): illustrations of working with geospatial data, including geopandas.
+* [`holoviews`](holoviews): illustrations of using HoloViews for convenient
+  visualizations.
+* [`pandas`](pandas): illustrations of using pandas and seaborn.
+* [`regexes`](regexes): illustrations of using regular expressions for validation
+  and information extraction from textual data.
+* [`web-scraping`](web-scraping): illustration of web scraping using beautiful soup
+  and graph representation using networkx.