Add documentation for Datasource constructor

[davidtheclark]

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It would be great to have documentation for the Datasource constructor, like the nice docs for other constructors.

[springmeyer]

Right, thanks for reminding of this! So, Mapnik's Datasource interfaces all accept key:value pairs and these are passed to Mapnik core "plugins". Each plugin tends to have custom values it accepts. They are detailed in this JSON doc https://github.com/mapnik/mapnik-reference/blob/gh-pages/3.0.6/datasources.json which is yet to be turned into nice visual documentation. So, the task that needs some help is:

• Make the datasources show up at http://mapnik.org/mapnik-reference/
• Do a pass over all the json data to make sure all the doc values are filled out
• Where there are gaps in the doc values, ticket missing ones at https://github.com/mapnik/mapnik-reference/issues and recruit @artemp, @GretaCB, or @mapsam to help fill them in by digging through a given plugins code and checking the C++ for the options it accepts: https://github.com/mapnik/mapnik/tree/master/plugins/input