Document bitfield usage in the users guide

[fitzgen]

Similar to the section on using unions, we should have a section on using bitfields.

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[ambaxter]

@highfive: assign me

[highfive]

Hey @ambaxter! Thanks for your interest in working on this issue. It's now assigned to you!

[fitzgen]

@ambaxter I don't think we have anything in CONTRIBUTING.md about using mdbook yet (another PR opportunity!), but between mdbook's documentation and running mdbook watch from the ./book directory, I think you should be able to figure everything out.

As far as content goes, we should just explain how bitfields get translated from C/C++ into Rust, and how to use them. You can experiment locally and look at the bitfield related tests. High bits are getters, setters, and bitfield unit constructors.

Let me know if you have any questions :)

[fitzgen]

@ambaxter making any progress here? Anything I can help with?

[ambaxter]

I am making some progress. A few things came up that took my time. Hope to be done in the next day or three.