Document min platforms supported in more detail

To be merged once Mapnik 3.x is used. Basically:

  - c++11 requirement means only recent linux is supported via binaries
  - OS X requirement of 10.9 is due to LTO (link time optimization) used to compile binaries. While I could drop this to be able to support 10.8 I'd prefer to target 10.9 unless a lot of people complain because it will allow faster and smaller binaries for Mapnik on >= 10.9
  - Not sure if the min Windows version has changed, so just guessing >= 7 is needed, but vista and XP might actually work

 - refs https://github.com/mapnik/node-mapnik/issues/324