Add docs on when to add new cargo features

[jeffcharles]

Description of the change

Adds documentation for when to add a new cargo feature to each crate when making a change.

Why am I making this change?

It's not clear when a new cargo feature should be added for a given crate.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.