add docs about geometrycolumns and crs for feature collections

docs/src/guides/developer.md

@@ -86,6 +86,22 @@ GeoInterface.geometrycolumns(::customcollection) = (:geometry,)  # can be multip
 
 The `geometrycolumns` enables other packages to know which field in a row, or column in a table, contains the geometry or geometries.
 
+It's important to note that the `geometrycolumns` should always return a `Tuple` of `Symbol`s.  However, it does have a fallback method
+that uses [DataAPI.jl](https://github.com/JuliaData/DataAPI.jl) metadata, if it exists, to retrieve the geometry columns.  This relies on
+the `GEOINTERFACE:geometrycolumns` metadata key.  GeoInterface.jl compatible writers may set this metadata key if writing to a format
+that does not have its own mechanism to store known geometry columns, like Arrow.
+
+Optionally, the `crs` method can also be implemented:
+```julia
+GeoInterface.crs(::customcollection)
+```
+
+This should return a `GeoFormatTypes.CoordinateReferenceSystem` type, such as `EPSG(code::Int)`, `WellKnownText(GeoFormatTypes.CRS(), wkt::String)`, 
+or `ProjString(p4::String)`.  See [GeoFormatTypes.jl](https://github.com/JuliaGeo/GeoFormatTypes.jl) for more information.
+
+The `crs` method also has a fallback that uses [DataAPI.jl](https://github.com/JuliaData/DataAPI.jl) metadata, if it exists, to retrieve the CRS.
+GeoInterface searches for the `GEOINTERFACE:crs` metadata key to retrieve the CRS.
+
 ## Geospatial Operations
 ```julia
 distance(geomtrait(a), geomtrait(b), a, b)