Documentation for `derive` feature.

[iMplode-nZ]

Currently, there is no documentation for the derive feature, which allows for easy implementations for Pod and Zeroable, which makes it difficult for people to effectively use the crate, especially as the two derive macros show up in the documentation.

Suggested fixes:

• Provide a note in the documentation suggesting use of the derive feature to get derive implementations for Pod and Zeroable (provide code example)
• Make the derive macro a default feature.

[Lokathor]

Additional docs with examples: yes, please do that PR.

Default feature: no. Pulling in the extra dependency runs up build times for all users, even the ones who never derive anything.