Add tip about commit.gpgsign to automate all commits being signed

[ottok]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Add tip about commit.gpgsign to automate all commits being signed

Context

The Debian Quality Assurance Salsa-CI requires all commits to be signed and I noticed that the last paragraph in the guide was missing this very relevant tip that is needed to actually have everyone working on the same repository to sign all commits.

[ottok]

@ben @petk Could you please review/merge this submission? It is trivial, should be quick to review.

[petk]

And now I think only sentence ending dot is missing.

[ottok]

Thanks for patience. Missing dot added.