See Section 2.4 in INSTALL.txt, it describes how to add an "about.html" file for the header.
“You can add a custom message to the top of the forum index / sub-forum
index pages by creating an 'about.html' file in the folder and
populating it with HTML code.
This feature is primarily designed for you to add a description at the
top of each forum to tell visitors what the forum is used for, rules
etc., but since any HTML can be used you can add whatever you want --
social buttons, links, images and so forth.
If you are using translations, then you can also provide translations
of 'about.html' to match by creating 'about_xx.html' files, where 'xx'
is the language code of the translation; see section 3 for information
about using and providing theme translations.”
Hope this helps.
Your friendly neighbourhood moderators: Kroc, Impressed, Martijn