Add crates.io badges

[japaric]

To all the crates that have been published to crates.io. The markdown to add is:

![crates.io](https://img.shields.io/crates/v/$CRATE.svg)

The result should look like this for $CRATE = l3gd20:

• L3GD20 - SPI - Gyroscope - Intro blog post -

[caemor]

I should be able to do that on Saturday/Sunday.

[caemor]

Can be closed?

[therealprof]

Sure. Haven't noticed that you didn't put a Fixes #1 in the PR description.