[DOCS] Add docs for runtime fields

docs/reference/mapping.asciidoc

@@ -13,7 +13,8 @@ are stored and indexed. For instance, use mappings to define:
 * custom rules to control the mapping for
   <<dynamic-mapping,dynamically added fields>>.
 
-A mapping definition has:
+A mapping definition includes metadata fields and fields, and can also include
+runtime fields:
 
 <<mapping-fields,Metadata fields>>::
 
@@ -30,6 +31,12 @@ document. Each field has its own <<mapping-types, data type>>.
 NOTE: Before 7.0.0, the 'mappings' definition used to include a type name.
 For more details, please see <<removal-of-types>>.
 
+<<runtime,Runtime fields>>::
+
+Runtime fields are not indexed, which saves disk space and makes data ingest
+faster. You can add fields to existing documents without reindexing your data
+and calculate field values dynamically at search time.
+
 [[mapping-limit-settings]]
 [discrete]
 === Settings to prevent mappings explosion
@@ -92,6 +99,7 @@ If your field mappings contain a large, arbitrary set of keys, consider using th
     `Long.MAX_VALUE` (no limit).
 
 [discrete]
+[[dynamic-mapping-intro]]
 == Dynamic mapping
 
 Fields and mapping types do not need to be defined before being used. Thanks
@@ -114,7 +122,7 @@ You can create field mappings when you <<create-mapping,create an index>> and
 
 [discrete]
 [[create-mapping]]
-== Create an index with an explicit mapping
+=== Create an index with an explicit mapping
 
 You can use the <<indices-create-index,create index>> API to create a new index
 with an explicit mapping.
@@ -262,3 +270,5 @@ include::mapping/fields.asciidoc[]
 include::mapping/params.asciidoc[]
 
 include::mapping/dynamic-mapping.asciidoc[]
+
+include::mapping/runtime.asciidoc[]