RTFM is a small mod adding a customizable, data-driven manual to the game that can be populated using Markdown files. It is aimed towards modpack makers that wish to add some in-game documentation using non-arcane formatting. The mod itself doesn't do anything useful unless you use it to actually add some pages to it, in particular it does not automatically add documentation for stuff in the game. It's just a system that allows you to document stuff yourself. Using Markdown files. That's it. Really.
Once installed and you've run Minecraft once, have a look at the config. It allows defining tabs for the manual (shown to the side of the manual GUI). These are essentially just glorified entry points to a specified page in the manual. You'll also find a default index file in the
rtfm subdirectory in your config directory, explaining how to add pages and such. Spoiler: it's really simple, just throw Markdown files in there and cross-reference them using standard Markdown links.
As a free bonus, due to the manual pages being Markdown, if you maintain them on Github or so, you automagically have a more or less perfect copy of your manual in web-browsable from (minus images of items).