Add check for BQ compatible schema-changes to PR review checklist

[fbertsch]

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This comes from the GCP schema handling doc.

We will need to initially start minimizing schema-incompatible changes for BQ. This only means that we can't change field types and can't remove fields. The former has historically (AFAIK) not been a common change, and the latter can be easily avoided by simply keeping the fields.

[jklukas]

Note on implementation: this test will likely need to pull the pre-PR state of any changed schemas via some git commands, then do some amount of parsing of the two versions of the schema to detect the conditions we want to avoid.

[fbertsch]

Those are good points @jklukas! I meant for this to just be added to the PR review checklist. Mark and I were discussing a longer-term CI test for schema changes, with a short-term improvement being akin to the one you mentioned, should we move that to a separate issue?

[jklukas]

should we move that to a separate issue

Indeed. I interpreted this as being about CI tests. Apologies for missing the scope. I agree that adding to a checklist is a good first step that can be done immediately.

[fbertsch]

Okay great. Updated the title of this issue to make it more clear, and let's move CI discussion to #317.