Action Details
Requirements and how-to articles for Steam's action:

Coming Soon.
You can broadcast live or pre-recorded content to your Steam product page (or pages). If properly setup, Steam will also show the 'Live' tag on your capsules throughout the Steam store. Popular Streams will also be surfaced on the Steam front page.
Coming Soon

Currently in Beta. May change at any time or may be discontinued.

You need this to get it done:

RTMP Streaming Specs (Audio)
Codec: AAC-LC
Maximum bitrate: 128 kbps
A game/application to stream. The account used for streaming must own the game you plan to stream (ie, see the game in your Steam library). The Steam Client cannot stream an unreleased game. However, you can use RTMP to stream an unreleased game if you want to give your audience a sneak peek of what you're working on.
RTMP Streaming Specs (Video)
Codec: h.264
Profile: High
Level: 4.1
Frame rate: 30FPS or 60FPS (Frames Per Second)
Aspect ratio: 16:9
Keyframe interval: 2 seconds
Maximum bitrate: 7000 kbps CBR (Constant Bit Rate)
(Note: By default, VMIX uses Profile Main and Level 3.0. You must change those settings in order for VMIX to stream correctly to Steam.)
App ID (if using external streaming software)
When using external software, Steam does not automatically know the game you are streaming. You can provide the App ID for the game you are streaming in the Broadcast Upload Settings so that your stream shows up on the game's product page.
Non-Limited Steam Account
Steam accounts for broadcasting must be non-limited. In other words, there must be at least $5 USD spent on Steam store with these accounts. Limited accounts will not be able to create an RTMP token and will not be able to broadcast onto the Steam platform using RTMP.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Steamworks Documentation - Setting Up A Stream
Information about streaming (setting up your application for broadcasting, configuring your stream and moderating chat, as well as general tips and rules.)
Link missing here or found errors? Improve How-Tos