docs: basic usage

This adds a basic 'usage' section to tell people how to do the most basic things with the firmware that comes with the badge.

docs/badge/usage.rst

+.. include:: <isonum.txt>
+
 Usage
 =====
 
-TODO
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+Hold your Flow3r with the pink part facing towards you, and the USB port facing
+upwards
+
+.. image:: overview.svg
+  :width: 700px
+
+Powering your Flow3r
+--------------------
+
+The Flow3r needs electricity to run - either from a battery or over its USB port.
+
+Once it has power available, you can turn it on by moving the right-hand side
+power switch (next to the 'flow3r' label on the front of the badge) towards the
+right.
+
+You should then see the badge spring to life and display 'Starting...' on the screen.
+
+Navigating the Menu
+-------------------
+
+The left shoulder button is used to navigate the menus of the badge. Moving it
+left and right selects an option in the menu. Pressing it down selects a menu
+option.
+
+The right shoulder button can be pressed down to quickly return 'back', either
+in a menu or an app.
+
+Dealing with Audio
+------------------
+
+The badge has two built-in speakers. Their loudness can always be adjusted by
+using the right shoulder button, left for lowering the volume and right for
+making it louder.
+
+You can plug in a pair of headphones to the 3.5mm jack on the bottom-left petal.
+The built-in speakers will then turn off and audio will go out through the
+headphones. You can adjust their volume in the same way.
+
+Showing your nick
+-----------------
+
+You can navigate to Badge |rarr| Nick to display your nick. If your nick is
+``flow3r``, congratulations! You're ready to go. Otherwise, you'll have to
+connect your badge to a computer and edit a file to change your nick.
+
+From the main menu, navigate to System |rarr| Disk Mode (SD). Connect your badge
+to a computer, and it will appear as a mass storage device (a.k.a. pendrive).
+Oen the file ```nick.json`` in a text editor and change your nick, the size of
+the font, and whatever else you wish.
+
+When you're done editing, unmount/eject the badge from your computer
+(``unmount`` on Linux is enough) and press the left shoulder button to exit Disk
+Mode. Then, go to Badge |rarr| Nick to see your changes!
+
+Playing Music
+-------------
+
+We ship some noise-making apps by default:
+
+shoegaze
+^^^^^^^^
+
+*TODO*
+
+Otamatone
+^^^^^^^^^
+
+*TODO*
+
+Simple Drums
+^^^^^^^^^^^^
+
+*TODO*
+
+Melodic
+^^^^^^^
+
+*TODO*
+
+Harmonic
+^^^^^^^^
+
+*TODO*
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