Document how vacant non-Google ESC seats get refilled

Extraordinary reelection in case an ESC member leaves before the current
term of the committee ends wasn't defined in the process documentation.

Change-Id: I1a1924ada2742e8d565904018a4aa94473806130
diff --git a/Documentation/dev-processes.txt b/Documentation/dev-processes.txt
index 5828cef..0eb3972 100644
--- a/Documentation/dev-processes.txt
+++ b/Documentation/dev-processes.txt
@@ -89,6 +89,15 @@
 already has dedicated seats in the steering committee (see section
 link:#steering-committee[steering committee]).
 
+If a non-Google seat on the steering committee becomes vacant before
+the current term ends, an exceptional election is conducted in order
+to replace the member(s) leaving the committee. The election will
+follow the same procedure as regular steering committee elections.
+The number of votes each maintainer gets in such exceptional election
+matches the number of seats to be filled. The term of the new member
+of the steering committee ends at the end of the current term of
+the steering committee when the next regular election concludes.
+
 [[contribution-process]]
 == Contribution Process