doc: update maintainers in README.md (cloudevents#478)

This commit modifies the structure of the README.md to add a markdown
heading. This change is motiviated by the CLO tool provided by CNCF.
This repository is failing a check for maintainers as you can see on
the dashboard:
https://clomonitor.io/projects/cloudevents/cloudevents#sdk-javascript

Documentation for this change can be found here:
https://github.com/cncf/clomonitor/blob/main/docs/checks.md#maintainers

Signed-off-by: Lance Ball <[email protected]>

Author: lance

README.md

@@ -212,12 +212,16 @@ There you will find Express.js, TypeScript and Websocket examples.
   to determine which week will have the call.
 - Slack: #cloudeventssdk channel under
   [CNCF's Slack workspace](https://slack.cncf.io/).
-- Maintainers typically available on Slack
-  - Lance Ball
-  - Lucas Holmquist
-  - Grant Timmerman
 - Email: https://lists.cncf.io/g/cncf-cloudevents-sdk
 
+## Maintainers
+
+Currently active maintainers who may be found in the CNCF Slack.
+
+- Lance Ball (@lance)
+- Lucas Holmquist (@lholmquist)
+- Grant Timmerman (@grant)
+
 ## Contributing
 
 We love contributions from the community! Please check the