If a new spec is not added to README, LintDiff won't validate it (related to Avocado).

Example:
- https://github.com/Azure/azure-rest-api-specs/pull/27899/checks?check_run_id=22018102665

It did not run on [`diagnostic.json`](https://github.com/Azure/azure-rest-api-specs/pull/27899/files#diff-566500bc779261d313ef55e941481497822c9191d8d1ef44c0a1b7b112d1a194)

In theory such scenario should be caught by `Swagger Avocado` (as can be seen [here](https://github.com/Azure/azure-rest-api-specs/pull/27899/checks?check_run_id=22018102823)), but we had to make it a non-blocking check.

For context, see [this Teams discussion](https://teams.microsoft.com/l/message/19:4f661242c446452e895359cc3ef45125@thread.tacv2/1709017067695?tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47&groupId=0cab4ce9-7691-42ae-82e3-460d4346a710&parentMessageId=1709017067695&teamName=ARM%20API%20Reviewers&channelName=ARM%20linter%20rules&createdTime=1709017067695).

This issue is very related to:
- https://github.com/Azure/azure-sdk-tools/issues/7776

Relevant doc:
- https://aka.ms/ci-fix#swagger-avocado