git: Add commit template

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

git-templates/commit-template.txt

@@ -0,0 +1,13 @@
+# Suricata commit template
+# See https://docs.suricata.io/en/latest/devguide/contributing/code-submission-process.html
+# Commit Title
+
+# Description
+# Provide a description of the changes in this commit that
+# gives the "why" for the change(s). Optionally, provide the
+# actual changes (the "what").
+# - List main changes
+# - List the motivation for each change
+# - Optionally, include the specific changes
+
+Ticket: <Redmine ticket number>