Add -S to git commit command in CONTRIBUTING.md

[emanuelb]

FROM CONTRIBUTING.md

3. Commit a failing test for the bug:

`git commit -am "Adds a failing test to demonstrate that thing"`

4. Commit a fix that makes the test pass:

`git commit -am "Adds a fix for that thing!"`

add -S to above commands, in order to apply signing, see issue #237

[kaplanlior]

What's the need to sign every commit ? Especially when most of them comes from the dev team contracted to work on the project.

238 should be enough in case you want to verify things for the long run.

[emanuelb]

you can read about this topic at (for better understanding): https://mikegerwitz.com/2012/05/a-git-horror-story-repository-integrity-with-signed-commits https://www.qubes-os.org/security/verifying-signatures/#how-to-verify-qubes-repos https://anarc.at/blog/2020-03-17-git-gpg-verification/