Add Alex Stewart as a maintainer.

Update contributing.rst to mention Alex
as one of the people who can be added
as a reviewer.

Change-Id: I30ff3e635e8c419e11e8f20394aaea5f284a10d5

docs/source/contributing.rst

@@ -111,8 +111,8 @@ Submitting a change to Ceres Solver
       git push origin HEAD:refs/for/master
 
    When the push succeeds, the console will display a URL showing the
-   address of the review. Go to the URL and add reviewers; typically
-   this is Sameer or Keir at this point.
+   address of the review. Go to the URL and add atleast one of the
+   maintainers (Sameer Agarwal, Keir Mierle, or Alex Stewart) as reviewers.
 
 3. Wait for a review.