Improve docs about NaiveDateTime conversion

[gerritsangel]

This adds some basic explanation on how to convert a NaiveDateTime to a DateTime.

Thanks for contributing to chrono!

• Have you added yourself and the change to the changelog? (Don't worry
  about adding the PR number)
• If this pull request fixes a bug, does it add a test that verifies that
  we can't reintroduce it?

[djc]

Thanks!

[djc]

Sorry for the slow review -- would you mind fixing up the lint issues found by CI and the changelog conflict?

[esheppa]

Thanks for these changes @gerritsangel - it looks like the lint failure is due to the changes in docs in lib.rs which are then automatically appended to the README.md. This is failed by the lint to prevent out of sync lib.rs docs and README.md being present in the repository.

If possible could you run ci/lint.sh locally and commit the changed README.md in this branch?