Be more explicit about extension code interdependencies

[jpmckinney]

http://ocds-standard-development-handbook.readthedocs.io/en/latest/tools/#ocds-extension-support lists tools that may need to be updated if changes are made to the behavior of extensions in OCDS.

We should also document:

• What reads extension_registry's compiled files? (add to extension_registry's readme)
• What relies on the format of entry-schema.json? Only the registry's own scripts rely on it
• What relies on the codelist CSV format?

As an example, I documented what depends on extension-schema.json here: https://github.com/open-contracting/standard-maintenance-scripts#extension-schemajson