Make Listening for Events section inclusive of custom controllers

[evansjohnson]

Description: As I understand, this section is really about listening for controller events, which won't necessarily be for buttons and axes. On an initial read through the docs, I had just read about creating custom controllers, so felt it may be more clear to make this section inclusive of the custom controllers just covered. Open to any suggested edits, I believe my statements are backed up from what I've seen so far but am generally unfamiliar with the library so need to be checked!

At a minimum, I'd like to fix up the "For each button, every time a button is pressed down, released, or for some cases, even touched." sentence fragment!

Changes proposed:

• Make section about listening to controller events, not specific to A-Frame controllers
• Link tracked-controls-component instead of relisting events

[evansjohnson]

thank you for your work with this library @dmarcos! it was really cool reading about how it builds on HTML and the event system already built into browsers

this is ready for another look if you get a chance and want to get it merged