📝 Add instructions for updating a package

[williln]

@joshuadavidthomas This is all based on your comment from directory. I tried to balance being specific about what the user would need to update against allowing for the flexibility that every package will likely be unique to some degree. Let me know if there is a section I should elaborate on more!

I did have a couple of questions:

• When there was a conflict in the settings or requirements, how did you decide which to keep? If you kept the non-template one, how did you know? Do you tend to leave a comment in the settings or requirements files so you know (example: # Pinning v0.0.00 because later versions don't work with other-package)
• For the token: I think your instructions mentioned at one point needing to pass the token into the copier prompts: Where should users go to obtain that token, or will they know?