ci: define permissions for contributing comment workflow

[fgreinacher]

Explicitely stating required permissions is considered best practice. This case was detected by Poutine, see
https://github.com/boostsecurityio/poutine/blob/main/docs/content/en/rules/default_permissions_on_risky_events.md.

⚒️ with ❤️ by Siemens

[github-actions]

Contributing to SWUpdate

Contributions are welcome ! Please follow the following guideline for
contributions.

Contribution Checklist

These are mostly general recommendations and are common practice in a
lot of FOSS projects.

• use git to manage your changes [recomended]

• follow as much as possible kernel codestyle [recomended]
  Nevertheless, some rules are not so strict as in kernel. The maximum
  line length can be extended over 80 chars if this increase code
  readability.

• add the required copyright header to each new file introduced
  [required]

• • add signed-off to all patches [required]

    • to certify the "Developer's Certificate of Origin", see
      below
    • check with your employer when not working on your own!

• • add version number for your patches if follow-up versions are
    requested [recomended]

    • Add a "Change from Vx" description under the commit message
      to take track of the history of the patch.
    • It is suggested to use excellent "patman" tool to manage
      patches series. This is part of U-Boot's project
      (tools/patman), but it can be used in other projects, too.

• check that your patches do not break build [required]

  • There is a set of configuration files in the
    configs/ directory. Please run a
    build for all files in the directory to ensure that SWUpdate is
    still buildable from configurations different as yours.

• • post patches to mailing list [required]

    • use git format-patch to
      generate your patches.
    • use git send-email if
      possible. This avoid corruptions due to the mailers
    • add a prefix [meta-swupdate] if patches are intended to
      the Yocto's meta layer.
    • send patches inline, do not append them
    • no HTML emails!

• do not use github Pull Request. github facilities are not used for
  this project. The review is done in a single place : the Mailing
  List. PR from github are ignored.

Patches are tracked by patchwork (see
http://jk.ozlabs.org/projects/patchwork/). You can see the status of
your patches at http://patchwork.ozlabs.org/project/swupdate/list.

Developer's Certificate of Origin 1.1

When signing-off a patch for this project like this

Signed-off-by: Random J Developer <[email protected]>

using your real name (no pseudonyms or anonymous contributions), you
declare the following:

By making a contribution to this project, I certify that:

1. The contribution was created in whole or in part by me and I
   have the right to submit it under the open source license
   indicated in the file; or
2. The contribution is based upon previous work that, to the best
   of my knowledge, is covered under an appropriate open source
   license and I have the right under that license to submit that
   work with modifications, whether created in whole or in part by
   me, under the same open source license (unless I am permitted to
   submit under a different license), as indicated in the file; or
3. The contribution was provided directly to me by some other
   person who certified (a), (b) or (c) and I have not modified it.
4. I understand and agree that this project and the contribution
   are public and that a record of the contribution (including all
   personal information I submit with it, including my sign-off) is
   maintained indefinitely and may be redistributed consistent with
   this project or the open source license(s) involved.