List prohibited field names

[duncandewhurst]

The documentation includes guidance on how to add additional fields. However, the schema prohibits certain field names on Geometry for compatibility with GeoJSON:

open-fibre-data-standard/schema/network-schema.json

Lines 1265 to 1266 in 25c3f87

	"propertyNames": {
	"pattern": "^(?!(properties$|^features$))"

We should list the prohibited field names in the documentation.