MarkDown

MarkDown is a plain-text language that uses a number of conventions to format text, add links, images, tables, etc in a concise and human-readable format. As such, MarkDown is frequently used to write software documentation. In a USGS code release, the README must use MarkDown, and the format is recommended for most other user documentation as well. (There are actually several different MarkDown styles, but we will exclusively refer to the CommonMark style, as this is the style supported by Gitlab and Github).

You can find a nice overview of MarkDown here: MarkDown Overview

And the complete specification for the language is here: MarkDown Spec

Gitlab extends MarkDown to support various other features, including flowcharts, Latex equations, and emojis. This extended version is known as Gitlab Flavored MarkDown (GLFM), and you can use these features to further enhance your documentation. You can find an overview of GLFM here: Gitlab Flavored MarkDown.