Document that Python 2 is required by the build

Change-Id: Ic03c3377950a9582779d3f0991eb270c4b00876b
diff --git a/Documentation/dev-bazel.txt b/Documentation/dev-bazel.txt
index 60a56dd..a613103 100644
--- a/Documentation/dev-bazel.txt
+++ b/Documentation/dev-bazel.txt
@@ -3,7 +3,7 @@
 [[installation]]
 == Installation
 
-You need to use Java 8 and Node.js for building gerrit.
+You need to use Python 2, Java 8, and Node.js for building gerrit.
 
 You can install Bazel from the bazel.io:
 https://www.bazel.io/versions/master/docs/install.html