Add basic documentation for beginners

[mbuesch]

There are many different traits and structs that interoperate in very complex ways in this crate. That makes it very hard to understand the basic concept of the crate. It's hard to find out where to look first in the documentation. As always, examples help a lot when explaining things.

This pull request adds a couple of basic examples to help kick starting the knowledge of people new to this project.

[mbuesch]

Thanks for your comments. I implemented all your suggestions. And I also did the following changes:

• Collapsed the supported device list. Now it takes less valuable first-sight screen space. But it's a bit harder to find a specific device in the list. What do you think? I think it's worth the trade off.
• Added a small section talking about crate features.

[Rahix]

Collapsed the supported device list. Now it takes less valuable first-sight screen space. But it's a bit harder to find a specific device in the list. What do you think? I think it's worth the trade off.

Agreed, I also prefer it like this. If people need to find a device, they can CTRL-F...

Added a small section talking about crate features.

Also a good idea! Thanks!