How to add many extra symbols

[chris-little]

This was originally part of Issue #25 which is now closed.

@tomkralidis What was the use case for this?

Obviously if WMO or ICAO standardise new symbols, change or deprecate existing ones, this repo can be updated with appropoirate Issues and Pull Requests.

If the smbols are national or private symbols, an obvious approach is to fork the repo and add symbols using the same style of SVG graphics and embedded metadata.