Add USB as socket to charging stations, some styling tweaks

assets/layers/charging_station/README.md

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+The charging station theme
+==========================
+
+As you might have noticed, the charging station theme is complicated and large.
+
+There are a ton of repititive questions. Luckily, we can generate those.
+
+If you want to add a missing socket type, then:
+
+- Add all the properties in 'types.csv'
+- Add an icon. (Note: icons are way better as pictures as they are perceived more abstractly)
+- Update license_info.json with the copyright info of the new icon. Note that we strive to have Creative-commons icons only (though there are exceptions)
+
+AT this point, most of the work should be done; feel free to send a PR. If you would like to test it locally first (which is recommended) and have a working dev environment, then run:
+
+- Run 'ts-node csvToJson.ts' which will generate a new charging_station.json based on the protojson
+- Run`npm run query:licenses` to get an interactive program to add the license of your artwork, followed by `npm run generate:licenses` 
+- Run `npm run generate:layeroverview` to generate the layer files
+- Run `npm run start` to run the instance