message: add splunk.com/on-call-routing-key docs to README
diff: ## Overview
  -This plugin displays Splunk On-Call, formerly VictorOps, information about an entity.
  +This plugin displays Splunk On-Call (formerly VictorOps) information associated with an entity.
  -There is a way to trigger an new incident directly to specific users or/and specific teams.
  +It also provides the ability to trigger new incidents directly to specific users or/and specific teams from within Backstage.
  -This plugin requires that entities are annotated with a team name. See more further down in this document.
  +This plugin requires that entities feature either a `splunk.com/on-call-team` or a `splunk.com/on-call-routing-key` annotation. See below for further details.
  This plugin provides:
  @@ -76,12 +76,22 @@ In addition, to make certain API calls (trigger-resolve-acknowledge an incident)
  ### Adding your team name to the entity annotation
  -The information displayed for each entity is based on the team name.
  -If you want to use this plugin for an entity, you need to label it with the below annotation:
  +The information displayed for each entity is based on either an associated team name or an associated routing key.
  +
  +To use this plugin for an entity, the entity must be labeled with either a `splunk.com/on-call-team` or a `splunk.com/on-call-routing-key` annotation.
  +
  +For example, by specifying a `splunk.com/on-call-team`, the plugin displays Splunk On-Call data associated with the specified team:
  +
  +```yaml
  +annotations:
  + splunk.com/on-call-team: <SPLUNK_ON_CALL_TEAM_NAME>
  +```
  +
  +Alternatively, by specifying a `splunk.com/on-call-routing-key`, the plugin displays Splunk On-Call data associated with _each_ of the teams associated with the specified routing key:
  ```yaml
  annotations:
  - splunk.com/on-call-team': <SPLUNK_ON_CALL_TEAM_NAME>
  + splunk.com/on-call-routing-key: <SPLUNK_ON_CALL_ROUTING_KEY>
  ```
  ### Create the Routing Key
  