Document third party verification flow + adapt coordinator tool docs

[soerenbf]

For refererence, this implements Concordium/concordium-governance-committee-voting#60

Changes

• Added --dev flag to readme for installing dependencies, as this was required to make the build work

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.