The BSON library has a DateTime wrapper type to override chrono::DateTime's default serde implementations. We should include documentation on how to use this and how to avoid common pitfalls of using chrono::DateTime directly.

example issues:

• https://github.com/mongodb/bson-rust/issues/121
• https://github.com/thedodd/wither/issues/56
• https://github.com/mongodb/bson-rust/issues/81
• https://github.com/mongodb/bson-rust/issues/65