Add instructions for excluding Pi-hole from Watchtower updates

[chippy01]

Added instructions to exclude Pi-hole container from Watchtower's auto-update system.

Description

Modified the README file in order to add the instructions to fully exclude the Pi-hole container from Watchtower.
The instructions can be find in Watchtower's official documentation Full exclude section

Motivation and Context

This change avoids a new Pi-hole user to go back on Watchtower's documentation to find how to exclude some container from the auto-update system.
The simple pice of setting that needs to be added on the Docker-compose is now available directly in the Pi-hole README

How Has This Been Tested?

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 not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[yubiuser]

Personally, I'd prefer to just link to the respective watchtower documentation on how to exclude a specific service from it. Our Readme is already quite long. I think linking to the right place of their documentation is already enough hand-holding.

[rdwebdesign]

I agree.

A link to the Full Exclude option is enough.

[chippy01]

Yeah, maybe just a link is enough, the watchtower guide for Full Exclusion is pretty straightforward, there's no need for a further explaination in the Pi-hole documentation.
As soon as i can i'll update the pull request.