CONTRIBUTING: add guidelines for commit msgs

CONTRIBUTING.md

@@ -1,5 +1,4 @@
-Contributing to coreutils
-=========================
+# Contributing to coreutils
 
 Contributions are very welcome, and should target Rust's master branch until the
 standard libraries are stabilized. You may *claim* an item on the to-do list by
@@ -13,4 +12,67 @@ following these steps:
 6. Remove utility from the to-do list in the README.
 7. Submit a pull request and close the issue.
 
-The steps above imply that, before starting to work on a utility, you should search the issues to make sure no one else is working on it.
+The steps above imply that, before starting to work on a utility, you should
+search the issues to make sure no one else is working on it.
+
+## Commit messages
+
+To help the project maintainers review pull requests from contributors across
+numerous utilites, the team has settled on conventions for commit messages.
+
+From http://git-scm.com/book/ch5-2.html:
+
+```
+Short (50 chars or less) summary of changes
+
+More detailed explanatory text, if necessary.  Wrap it to about 72
+characters or so.  In some contexts, the first line is treated as the
+subject of an email and the rest of the text as the body.  The blank
+line separating the summary from the body is critical (unless you omit
+the body entirely); tools like rebase can get confused if you run the
+two together.
+
+Further paragraphs come after blank lines.
+
+  - Bullet points are okay, too
+
+  - Typically a hyphen or asterisk is used for the bullet, preceded by a
+    single space, with blank lines in between, but conventions vary here
+```
+
+Furthermore, here are a few examples for a summary line:
+
+* commit for a single utility
+
+```
+nohup: cleanup and refactor
+```
+
+* commit for a utility's tests
+
+```
+tests/rm: test new feature
+```
+
+Beyond changes to an individual utility or its tests, other summary
+lines for non-utility modules include:
+
+```
+README: add help
+```
+
+```
+travis: fix build
+```
+
+```
+uucore: add new modules
+```
+
+```
+uutils: add new utility
+```
+
+```
+gitignore: add temporary files
+```