Add undocumented field argument

schemas/README.md

@@ -170,6 +170,7 @@ Supported keys to describe fields
 - 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): Adds a beta marker for the field to the description. The text provided in this attribute is used as content of the beta marker in the documentation. Note that when a whole field set is marked as beta, it is not necessary nor recommended to mark all fields in the field set as beta. Beta notices should not have newlines.
+- parameters (optional): An object of arbitrary parameters to include in the field settings.
 
 Supported keys to describe expected values for a field