git: Add commit template

[jlucovsky]

Issue: none

This commit adds a template that identifies commit message elements that we find important. The Suricata development team requests that contributions use the template to help improve commit messages. We reserve the right to strictly enforce adherence to the template in the future.

Configure git to use this template with: git config commit.template ..github/commit-template.txt

Describe changes:

• Add commit template with instructions on how to configure

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request, link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_REPO= SV_BRANCH= SU_REPO= SU_BRANCH= LIBHTP_REPO= LIBHTP_BRANCH=