Adding initial event schema documentation

src/main/resources/org/opensearch/ubi/MAPPINGS.md

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+# Event Schema
+## Primary fields include:
+- `action_name` - any name you want to call your event 
+- `timestamp` - should be set automatically
+- `user_id`. `session_id`, `page_id` - are id's largely at the calling client's discretion for tracking users, sessions and pages 
+- `query_id` - ID for some query.  Note that it could be a unique search string, or it could represent a cluster of related searches (i.e.: *dress*, *red dress*, *long dress* could all have the same `query_id`).  Either the client could control these, or the `query_id` could be retrieved from the API's response headers as it keeps track of queries on the node
+- `message_type` - originally thought of in terms of ERROR, INFO, WARN...but could be anything useful such as `QUERY` or `PURCHASE` 
+- `message` - optional text for the log entry
+![event mapping schema](./event_mapping.png)
+
+## Other fields & data objects
+- `event_attributes` - contains various, common attributes associated with many user events
+- `event_attributes.data` - contains an associated JSONified data object (i.e. products, user info, etc) if there are any
+- `event_attributes.data.data_type` - indicates the type/class of object
+- `event_attributes.data.data_id` - points to a unique id representing and instance of that object
+- `event_attributes.data.description` - optional description of the object
+- `event_attributes.data.transaction_id` - optionally points to a unique id representing a successful transaction
+- `event_attributes.data.to_user_id` - optionally points to another user, if they are the recipient of this object
+- `event_attributes.data.data_detail` - optional data object/map of further data details