Update README with Slack instructions (cartography-cncf#1381)

Updates the readme to the new CNCF Slack workspace.

Signed-off-by: Alex Chantavy <achantavy@lyft.com>
Signed-off-by: chandanchowdhury <chandan.chowdhury@hotmail.com>

README.md

@@ -80,11 +80,10 @@ Directly querying Neo4j is already very useful as a sort of "swiss army knife" f
 
 ## Community
 
-- Join us on `#cartography` on the [Lyft OSS Slack](https://join.slack.com/t/lyftoss/shared_invite/enQtOTYzODg5OTQwNDE2LTFiYjgwZWM3NTNhMTFkZjc4Y2IxOTI4NTdiNTdhNjQ4M2Q5NTIzMjVjOWI4NmVlNjRiZmU2YzA5NTc3MmFjYTQ).
+- Hang out with us on Slack: Join the CNCF Slack workspace [here](https://communityinviter.com/apps/cloud-native/cncf), and then join the `#cartography` channel.
 - Talk to us and see what we're working on at our [monthly community meeting](https://calendar.google.com/calendar/embed?src=lyft.com_p10o6ceuiieq9sqcn1ef61v1io%40group.calendar.google.com&ctz=America%2FLos_Angeles).
   - Meeting minutes are [here](https://docs.google.com/document/d/1VyRKmB0dpX185I15BmNJZpfAJ_Ooobwz0U1WIhjDxvw).
   - Recorded videos are posted [here](https://www.youtube.com/playlist?list=PLMga2YJvAGzidUWJB_fnG7EHI4wsDDsE1).
-- Our current project roadmap is [here](https://github.com/orgs/lyft/projects/26/views/1).
 
 ## License