Adds footer to signed contract.

![boom](https://media1.giphy.com/media/TR95rSgPsHqiQ/200w.gif)
This is inside an if branch that just checks if there is a leading `-`, so this doesn't check for the end marker. 

We should check for the complete end marker

See https://github.com/monetas/protocol-docs/blob/master/content/encoding/SectionFormat.md