Add a LICENSE

[darkexplosiveqwx]

Thank you for your contribution to the Pi-hole Community!

Please read the comments below to help us consider your Pull Request.

We are all volunteers and completing the process outlined will help us review your commits quicker.

Please make sure you

1. Base your code and PRs against the repositories developmental branch.
2. Sign Off all commits as we enforce the DCO for all contributions
3. Sign all your commits as they must have verified signatures
4. File a pull request for any change that requires changes to our documentation at our documentation repo

---

What does this PR aim to accomplish?:

provide clarity about the licensing of PADD
I'm going with EUPL 1.2, as the rest of Pi-hole uses it.

How does this PR accomplish the above?:

Adds a LICENSE, in similar style as core, FTL and web
Adds the license header, similar to core and FTL.

Technically, when PADD had no LICENSE file it counts as "All rights reserved".
Therefore, this is considered a re-license and needs approval from all contributors.

Fron git commit metadata:

$ git shortlog -s -n -e
   168  Christian König <ckoenig@posteo.de>
   141  yubiuser <ckoenig@posteo.de>
   108  yubiuser <github@yubiuser.dev>
    59  Jim McKenna <jim@jpmck.com>
    38  dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
    25  DL6ER <dl6er@dl6er.de>
    23  Timothy Johnstone <timothy.g.johnstone@gmail.com>
    19  RD WebDesign <github@rdwebdesign.com.br>
    14  Dan Schaper <dan.schaper@pi-hole.net>
    13  Adam Warner <me@adamwarner.co.uk>
     8  Subhaditya Nath <sn03.general@gmail.com>
     7  darkexplosiveqwx <101737077+darkexplosiveqwx@users.noreply.github.com>
     6  DL6ER <DL6ER@users.noreply.github.com>
     6  Michael Woolweaver <michael@woolweaver.bid>
     6  thomasmerz <thomasmerz@users.noreply.github.com>
     5  Christian Boyle <mail@christianboyle.com>
     4  KnownAim <kevorkterterian@gmail.com>
     4  Thomas Merz <git@rhoenwurz.de>
     4  fullburnen <fullburnen@protonmail.com>
     4  pintomax <massimiliano.pinto@gmail.com>
     4  pralor-bot <pralor-bot@users.noreply.github.com>
     3  Dominik <DL6ER@users.noreply.github.com>
     3  Peter Galantha <peter.galantha@gmail.com>
     2  Christian Boyle <christianboyle@users.noreply.github.com>
     2  Kerdic <jimmy.baldwin@gmail.com>
     1  Aaron Tiensivu <atiensivu@users.noreply.github.com>
     1  Adam Warner <github@adamwarner.co.uk>
     1  Anthony <m600x@users.noreply.github.com>
     1  Blumlaut <13604413+Blumlaut@users.noreply.github.com>
     1  Bucking-Horn <buckhorn@weibsvolk.org>
     1  Collin O'Connor <collin.d.oconnor@gmail.com>
     1  FauFra <frasca.fausto@gmail.com>
     1  Lenoard Lee <sheeeng@gmail.com>
     1  Richard Kuhnt <r15ch13+git@gmail.com>
     1  Rob Gill <rrobgill@protonmail.com>
     1  Subhaditya Nath <56714626+subnut@users.noreply.github.com>
     1  Tom Morgan <tom472@gmail.com>
     1  XhmikosR <xhmikosr@gmail.com>
     1  Zack Kitzmiller <zackkitzmiller@users.noreply.github.com>
     1  danmurry <44685332+danmurry@users.noreply.github.com>
     1  darkexplosiveqwx <the.qwx.channel@gmail.com>
     1  fullburnen <70560944+fullburnen@users.noreply.github.com>
     1  strangelookingnerd <49242855+strangelookingnerd@users.noreply.github.com>
$ git log | grep -i "Co-authored-by" | sort -u
    Co-authored-by: Adam Warner <github@adamwarner.co.uk>
    Co-authored-by: Adam Warner <me@adamwarner.co.uk>
    Co-authored-by: Dan Schaper <dan.schaper@pi-hole.net>
    Co-authored-by: DL6ER <DL6ER@users.noreply.github.com>
    Co-authored-by: k-rister <k-rister@users.noreply.github.com>
    Co-authored-by: RD WebDesign <github@rdwebdesign.com.br>
    Co-authored-by: Scott Smith <scottsm@mac.com>
    Co-authored-by: yubiuser <ckoenig@posteo.de>
    Co-authored-by: yubiuser <github@yubiuser.dev>

I think it would be good to have as many contributors as possible agree to this change and possibly mention the ones not explicitly agreeing in LICENSE (similar as it has been done with core and web)

Should I go ahead and ping everyone's GitHub for this?

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

---

• I have read the above and my PR is ready for review. Check this box to confirm

[darkexplosiveqwx]

I think we should leave this open for a couple weeks to give contributors some time to explicitly approve or deny the re-license.