document beta field

schemas/README.md

@@ -133,6 +133,7 @@ Supported keys to describe fields
 - format: Field format that can be used in a Kibana index template.
 - normalize: Normalization steps that should be applied at ingestion time. Supported values:
   - array: the content of the field should be an array (even when there's only one value).
+- beta (optional): If `True`, the field will be marked in the documentation with a `beta` label.
 
 Supported keys to describe expected values for a field
 
@@ -151,7 +152,7 @@ Supported keys to describe expected values for a field
   Optionally, entries in this list can specify 'expected\_event\_types'.
 - expected\_event\_types: list of expected "event.type" values to use in association
   with that category.
-  
+
 Supported keys when using the [alias field type](https://www.elastic.co/guide/en/elasticsearch/reference/current/alias.html)
 
 ```YAML