3.1 entry for mapnik-reference

[springmeyer]

To support Mapnik 3.1.x (current master and future release) I think we'll need a new directory for this version.

@talaj - you've been most active working on the rendering side of future Mapnik 3.1 (current master). Are there any new styling properties or changes to styling properties? (I don't see any at https://github.com/mapnik/mapnik/wiki/API-changes-between-v3.0-and-v3.1).

To support Mapnik 3.1 can we simply copy the 3.0.6 directory?

/cc @artemp

[talaj]

Are there any new styling properties or changes to styling properties?

I know of these: mapnik/mapnik#3715, mapnik/mapnik#3782, mapnik/mapnik#3512, mapnik/mapnik#3596, mapnik/mapnik#3474.

[springmeyer]

great, thanks for calling those out, I've created mapnik/mapnik#3787 to track getting them documented.