Import wiki page on commit messages

https://wiki.civicrm.org/confluence/display/CRMDOC/Git+Commit+Messages+for+CiviCRM

This isn't exactly an import. More of a re-write.

docs/tools/git.md

@@ -95,7 +95,23 @@ __TODO__
 
 ### Committing {:#committing}
 
-__TODO__
+Guidelines for commit messages:
+
+* The first line should be a meaningful **subject**, which should:
+    * be prefixed with a Jira issue number (if the commit is to CiviCRM core)
+    * mention a "subsystem" after the issue number
+    * be 72 characters or less, in total
+    * be in "Sentence case"
+    * use the imperative mood
+    * not end in a period
+    * examples: 
+        * `CRM-20600 - Civi\Angular - Generate modules via AssetBuilder`
+        * `CRM-19417 - distmaker - Change report to JSON`
+* (optionally) After the subject, include a short **body**, which should:
+    * have a blank line above it (below the subject)
+    * be wrapped at 72 characters
+    * explain *what*, *why*, and *how*
+
 
 
 ### Submitting a pull request {:#pr}

redirects/wiki-crmdoc.txt

@@ -173,3 +173,4 @@ Dedupe+and+merge+testing testing/manual/#dedupe
 Smart+group+testing testing/manual/#smart-group
 Tarball+installation+testing testing/manual/#tarball
 Contributing+to+CiviCRM+using+GitHub tools/git/#github
+Git+Commit+Messages+for+CiviCRM tools/git/#committing