Adding example for signing commits by jarrodek · Pull Request #1740 · nodegit/nodegit

This is related to #1018

Adds an example to demonstrate how to use signed commits using GPG key, nodegit, and openpgp.

It adds a dummy GPG key in examples/gpg-keys/secret-key-489B1963.asc and signing-commits.js with each part of the operation defined in it's own function for readability.

I believe it will be useful as there is no guide of how to sign commits using nodegit, nor any community content related to this matter.