How to make Android 6.0 Marshmallow show the battery percentage in the status bar
First of all, you need to enable Android Marshmallow's System UI Tuner (here's how to do it). Once that's enabled, you'll find the System UI Tuner sub-menu in your device's main Settings menu, at the very bottom of it. Taping on that will provide access to various things, one of it being the ability to show embedded battery percentage in the status bar - you just have to tap on the corresponding toggle to enable it. And that's it, now you'll see the battery percentage on your Android 6.0 Marshmallow device at any time when the status bar is visible, except for when the device is charging.
Of course, some non-stock Android phones and tablets (like Samsung's Galaxies) have had a similar option for quite some time. But it's nice to see this implemented in stock Android, too.