Add changelog entry for traffic monitor astats csv support

[ezelkow1]

What does this PR (Pull Request) do?

Adds changelog entry for tm astats csv support

• This PR fixes #REPLACE_ME OR is not related to any Issue

Which Traffic Control components are affected by this PR?

• Documentation

What is the best way to verify this PR?

If this is a bug fix, what versions of Traffic Control are affected?

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[zrhoffman]

Changelog entry looks good