[CONTRIBUTING] Add .git-commit-template

Meant to help contributors follow our guidelines. This will close
ROOT-8915 on JIRA.

.git-commit-template

@@ -0,0 +1,17 @@
+# Commit summary: Written in the present imperative mood.
+# Prefixed by a [scope] tag, not exceeding 50 characters (excl. scope tag).
+#                                                # <- 50 chars is here.
+[scope] Add/fix/improve/... X
+
+# Commit message: Describe the 'why' behind the change. Wrap at 72 characters.
+#                                                                      # <- 72 chars is here.
+
+
+# In case the commit fixes a specific issue, uncomment and add the issue ID:
+# Fixes https://github.com/root-project/root/issues/ISSUE_ID
+
+# Tell git to use this commit template by adding it to the repository configuration:
+#   git config commit.template .git-commit-template
+
+# See https://github.com/root-project/root/blob/master/CONTRIBUTING.md for more
+# information on how to contribute to ROOT.

CONTRIBUTING.md

@@ -56,6 +56,14 @@ It should describe the **why**, rather than the **what** and **how** (we can gat
 change diff, respectively).
 The commit message should be wrapped at 72 characters.
 
+> [!TIP]
+> We provide a commit message template to help with following the above guidelines. It can be found in the root of this
+> repository as [`.git-commit-template`](https://github.com/root-project/root/blob/master/.git-commit-template),
+> and can be set to automatically be used for every commit with the following command:
+> ```sh
+> $ git config commit.template .git-commit-template
+> ```
+
 ## Your Pull Request
 
 > [!NOTE]