Merge pull request #449 from facebookresearch/pr/commit

add more guidelines for the one-line summary of commit messages

Author: ftynse

CodingConventions.md

@@ -594,6 +594,8 @@ A commit message consists of a one-line summary, followed by an empty line and
 the main body with more details focusing on the motivation of the change.
 Both the one-line summary and the lines in the main body
 should not exceed 72 characters.
+The one-line summary should use an imperative and
+should not have a closing period.
 The commit message should contain all the information that is needed
 to understand the change within the git repository.
 The main body can only be omitted if the motivation is completely obvious