Add instructions for converting Shapefiles

app/broadcast_areas/source_files/README.md

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+# Adding or updating source files 
+
+If the files are in GeoJSON format you can drop them straight in.
+
+If you have a Shapefile, for example from the [ONS Open Geography Portal](https://geoportal.statistics.gov.uk/), you need to convert it:
+- download the .zip from the portal
+- extract it, giving you a folder with a few files in
+- go to https://mapshaper.org
+- drag all the files in the folder into mapshaper
+- click ‘import’
+- you should see a rendering of the shapes 
+- click ‘console’ (top right)
+- run this command: `mapshaper -proj wgs84`
+- click ‘export’ (top right)
+- choose ‘GeoJSON’
+- click the button
+- done