Title

Add info about --sync-without-deleting to the docs

Motivation and Context

I'm new here, wanted to sync my libraries and got and idea "i should just run cronjob with sync command. Gosh, docs say that it deletes tracks thart are no longer in playilsts and i know that spotify sometimes deletes the tracks just because. Started to dig through the sync command code - found this (undocumented except in --help) flag.

How Has This Been Tested?

n/a

Screenshots (if appropriate)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation enchancement

Checklist

• [n/a] My code follows the code style of this project
• [n/a] My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have read the CORE VALUES document
• [n/a] I have added tests to cover my changes
• All new and existing tests passed