docs(CONTRIBUTING.md): add a note about subject tense and case

The contribution guidelines previously did not mention that the commit
messages should be lowercase and in the past tense. This has been
updated with an example.

Author: Gary Rennie

CONTRIBUTING.md

@@ -18,6 +18,16 @@ All pull request commit messages are automatically checked using [GitCop](http:/
 
 No line in your commit message shoult be longer than **72** characters. If you have something that is longer than this (such as a url), please put it on to multiple lines with a backspace at the end of each line.
 
+##### Subject
+
+The subject (first commit line) should be written in the past tense and should start with a lower case letter. e.g.
+
+    feat(Changex.Formatter): add ability to determine current_version
+
+Instead of:
+
+    Feat(Changex.Formatter): Add ability to determine current_version
+
 ##### Format
 
 Because Changex automatically generates the [CHANGELOG.md](https://github.com/Gazler/changex/blob/master/CHANGELOG.md) from the commit messages, it is important that the follow these rules.