Added section about Steam.

docs/api/intro.md

@@ -16,16 +16,19 @@ perhaps more familiarly as `namespaces`.
 The Blitz Runtime Libraries (`BRL`), contain the core of BlitzMax functionality that you
 are likely to use from day to day.
 
-The `PUB` modscope generally contains third party libraries and platform-specific
-helpers. These are considered more low-level, and generally you won't use
-these modules directly.
-
 `MaxGUI` provides access to native windowing toolkits, such as Cocoa on macOS
 and GTK+3 on Linux, allowing you to build complex user interface-driven
 applications.
 
+The `PUB` modscope generally contains third party libraries and platform-specific
+helpers. These are considered more low-level, and generally you won't use
+these modules directly.
+
 The `SDL` modscope can be used as a replacement graphics and system-integration
 backend for the default modules provided by `BRL`. It is optional on the main
 desktop platforms, but generally required on the other targets (iOS, Android, rpi, NX).
 It is a full implementation of SDL in BlitzMax, including SDL's Render and Window subsystems,
 which can be used directly if preferred.
+
+The `Steam` modscope is for integrating your game with Steam, providing access to all the commonly used Steam APIs,
+such as friends, leader boards and Steam Workshop.