Add FAQ about known issue (503 with concurrent requests)

[jasminejaksic]

As a follow-up to 1038# please add FAQ that lets the users know that they can encounter 503s if they run concurrent threads

[andraxylia]

@jasmine-jaksic to clarify, this is only related to bookinfo, not a general problem.

[stale]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 2 weeks unless it is tagged "help wanted" or other activity occurs. Thank you for your contributions.

[andraxylia]

Not needed.