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/