You can add a text, like an instruction, your contact details or a link to further information in the header and footer of the signing page. You'll find the settings, which applies to the entire account on Account Settings -> Settings.
You style your text and add links using the formatting language Markdown. Learn more about the basics of Markdown.
To make a link you can write:
[text to show](link)
For italics, write:
For bold, write: