Update external-library.md

[smy213]

Add a note about current restrictions when sharing.

Description

Summing up from this discord: https://discord.com/channels/979116623879368755/1394413627108229170
This note would have helped me a lot. When starting with Immich it is difficult to bring the concepts of sharing together...

Fixes # (issue)

How Has This Been Tested?

n/a

Screenshots (if appropriate)

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

[github-actions]

Label error. Requires exactly 1 of: changelog:.*. Found: . A maintainer will add the required label.