adding normative statement about source information

[kathryn-ods]

Overview

• What does this pull request do? adds a normative requirement that statements should include source info
• How can a reviewer test or examine your changes?
• Who is best placed to review it?

Closes #537

Translations

• [ ] New or edited strings appearing as a result of this PR will be picked up for translation
• [ ] I've notified the translation coordinator of any new strings that will need translating. See: https://openownership.github.io/bods-dev-handbook/translations.html

Documentation & Release

• [ ] I've checked that the documentation builds without failing. See: https://github.com/openownership/data-standard#building-the-documentation-locally
• [ ] I've thought about how and when this needs to be released. See: https://openownership.github.io/bods-dev-handbook/standard_releases.html
• [ ] I've updated the changelog, if necessary.
• [ ] I've updated reference.rst to reflect any changes to schema structure or naming.
• [ ] I've added or edited any other related documentation. See: https://github.com/openownership/bods-dev-handbook/blob/master/style_guide.md

[kathryn-ods]

@kd-ods imo this is the only thing in the sources and annotations page that isn't otherwise covered in the schema/docs. See comments in #537 for more detail.

[kathryn-ods]

@kd-ods have made those changes and made #638 for the annotations issue