Document .git/base_sha in README

Author: Alex Conrad

README.md

@@ -134,6 +134,8 @@ git config --get pullrequest.userlogin  # returns the github user login for the
 
  * `.git/base_branch`: the base branch of the pull request
 
+ * `.git/base_sha`: the commit of the base branch of the pull request
+
  * `.git/userlogin`: the user login of the pull request author
 
  * `.git/head_sha`: the latest commit hash of the branch associated with the pull request