More details in readme

[brozorec]

1. Add context for the upgrade-related checks by referencing the Upgrades SEP: The readme should mention the Upgrades SEP and explain that the upgrade-related checks follow the recommendations outlined in it. This helps clarify the rationale behind the validations.
2. Clarify defaults for network and source account: In the section # You can also specify network and source (optional):, document what defaults are used if source_account or network are not provided.