How To Use Android's New DistractionFree Reading Mode
You can have a main shortcut, a hotkey, or both, so choose the options that work best for you. If you click More options next to the Accessibility button, you can customize the button's on-screen appearance with options for its size and transparency.
Reading mode can also be activated in quick settings: swipe down with two fingers from the top of the screen to see it. Click on the pencil icon and you will see the available reading mode panel which you can drag to the quick settings bar.
How to get started with Reading mode

You can activate reader mode whenever and wherever you are on Android - just press the quick settings button or square, or use the volume key combination. If Reader mode fails to process the contents of the screen, a message will be displayed.
Reading mode works best when browsing the web with text, not games or images, for example. A new cover will appear on your screen and at the top you will see an estimate of how long it will take you to complete the current article - start scrolling down to read it.
If you don't like the text and colors, you can change them. Tap the text button (two T's) in the lower right corner and you can quickly increase or decrease the size of the text on the screen. Click the gear button in the lower left corner to open more options, including text size, style, alignment, and color.
Click the Play button below the text to have it read aloud to you. Navigation arrows appear: press to skip a sentence, or long press to skip a paragraph. You will also see other icons that allow you to change other settings such as playback speed (the player on the right) and playback volume (the gear on the left).
When you're done with Reading Mode, swipe down on the down arrow at the top of the screen to return to normal Android Mode. There are not many advanced features, but it is indeed a very useful tool.