Clarify safety and ease of use

[johnbradley]

In PR #66 a concern was raised about content saying pybioclip doesn't require ML knowledge to run. The concern is users might take this to mean they do not need to understand the limitations and biases present in the underlying model.
See #66 (comment) for more.

Two places may need adjustment:

• README

  ... No particular understanding of ML or computer vision is required to use it. ...

• docs/index.md

  No particular coding knowledge of ML or computer vision is required to use pybioclip.