Describe ItemCollection and CollectionCollection (#1290)

Co-authored-by: Matthias Mohr <matthias@mohr.ws>

collection-spec/README.md

@@ -44,3 +44,7 @@ a collection and be able to adapt it to most any data model.
 
 Implementors are encouraged to do what makes sense for them, and to check out the [examples](../examples/) and 
 [other implementations](https://stacindex.org/catalogs) for current best practices.
+> \[!NOTE]
+> In cases where a set of Collections is grouped together (e.g. for providing search results),
+> the [CollectionCollection](https://github.com/radiantearth/stac-api-spec/blob/main/fragments/collectioncollection/README.md)
+> object has been defined in the STAC API specification.

item-spec/README.md

@@ -21,3 +21,12 @@ schemas validate additional fields defined in *[Common Metadata](common-metadata
 
 **Common Metadata:** A set of commonly-used fields for STAC Items is listed in 
 *[common-metadata.md](common-metadata.md)*.
+
+> \[!NOTE]
+> In cases where a set of Items is grouped together (e.g. for providing search results),
+> the [ItemCollection](https://github.com/radiantearth/stac-api-spec/blob/main/fragments/itemcollection/README.md)
+> object has been defined in the STAC API specification.
+> 
+> Similarly to the relationship between a GeoJSON Feature and a STAC Item,
+> a STAC ItemCollection should be a valid GeoJSON [FeatureCollection](https://tools.ietf.org/html/rfc7946#section-3.3)
+> to allow interoperability with existing tools that support GeoJSON.

item-spec/item-spec.md

@@ -335,7 +335,7 @@ that talks about common use cases of additional fields for assets.
 ## Media Type for STAC Item
 
 A STAC Item is a GeoJSON file ([RFC 7946](https://tools.ietf.org/html/rfc7946)), and thus should use the 
-[`application/geo+json`](https://tools.ietf.org/html/rfc7946#section-12) as the [Media Type](https://en.wikipedia.org/wiki/Media_type) 
+[`application/geo+json`](https://tools.ietf.org/html/rfc7946#section-12) as the [Media Type](https://en.wikipedia.org/wiki/Media_type)
 (previously known as the MIME Type). 
 
 ## Extensions