.gitmessage: Added a commit template

Helps to write commit messages, that are conforming to CONTRIBUTING.md.

.gitmessage

@@ -0,0 +1,19 @@
+# Title: Summary, imperative, start upper case, don't end with a period
+# No more than 50 chars. #### 50 chars is here:  #
+
+# Remember blank line between title and body.
+
+# Body: Explain *what* and *why* (not *how*).
+# Wrap at 72 chars. ################################## which is here:  #
+
+# At the end: Include Co-authored-by for all contributors. 
+# Include at least one empty line before it. Format: 
+# Co-authored-by: name <[email protected]>
+
+# 1. Separate subject from body with a blank line
+# 2. Limit the subject line to 50 characters
+# 3. Capitalize the subject line
+# 4. Do not end the subject line with a period
+# 5. Use the imperative mood in the subject line
+# 6. Wrap the body at 72 characters
+# 7. Use the body to explain what and why vs. how