Spec: add a note about mapping of Parquet BYTE_ARRAY type to Arrow ty…

…pes (#190)

Fixes #187

format-specs/geoparquet.md

@@ -13,6 +13,9 @@ This is version 1.1.0-dev of the GeoParquet specification.  See the [JSON Schema
 ## Geometry columns
 
 Geometry columns MUST be stored using the `BYTE_ARRAY` parquet type. They MUST be encoded as [WKB](https://en.wikipedia.org/wiki/Well-known_text_representation_of_geometry#Well-known_binary).
+
+Implementation note: when using the ecosystem of Arrow libraries, Parquet types such as `BYTE_ARRAY` might not be directly accessible. Instead, the corresponding Arrow data type can be `Arrow::Type::BINARY` (for arrays that whose elements can be indexed through a 32-bit index) or `Arrow::Type::LARGE_BINARY` (64-bit index). It is recommended that GeoParquet readers are compatible with both data types, and writers preferably use `Arrow::Type::BINARY` (thus limiting to row groups with content smaller than 2 GB) for larger compatibility.
+
 See the [encoding](#encoding) section below for more details.
 
 ### Nesting