Document known issue - "Source name already exists in Event Notifications instance"

[ocofaigh]

Document this known issue: IBM-Cloud/terraform-provider-ibm#5579 in the readme until its fixed by SCC.

[ocofaigh]

Not going to document - the issue was fixed in provider 1.71.3 (its now required to pass source name). There is an effort under way to update SCC DA with this fix and add it to RAG stack