Add William Rucklidge to the list of maintainers.

Update contributing.rst.

Change-Id: I4e9712d143f15301abfdc49ca938abc042a29ddd

docs/source/contributing.rst

diff --git a/docs/source/contributing.rst b/docs/source/contributing.rst
index b07e685..b8f8165 100644
--- a/docs/source/contributing.rst
+++ b/docs/source/contributing.rst
@@ -103,7 +103,8 @@
 
    When the push succeeds, the console will display a URL showing the
    address of the review. Go to the URL and add at least one of the
-   maintainers (Sameer Agarwal, Keir Mierle, or Alex Stewart) as reviewers.
+   maintainers (Sameer Agarwal, Keir Mierle, Alex Stewart or William
+   Rucklidge) as reviewers.
 
 3. Wait for a review.