diff --git a/Makefile b/Makefile
index 222aa0a..9c74607 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 NAME = verypossible/serverless
-VERSION = 1.19.0
+VERSION = 1.21.0
 
 .PHONY:	all py2 py3 shell
 
diff --git a/README.md b/README.md
index 19fda4c..edb335f 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,14 @@
 # serverless
+
 Automated Docker build for the Serverless framework
+
+There are two `Dockerfile`s which are nearly the same.  One uses Python 2 base image and the other
+Python 3.
+
+These images are build from Docker Cloud.  The Serverless version to build is specified as a build
+`ARG` (build argument) which is set using `--build-arg` during the `docker build` command.  To pass
+this argument, we need a Docker Cloud build hook which resides in `hooks/build`. 
+
+See the following for more information on Docker Cloud build hooks:
+
+https://docs.docker.com/docker-cloud/builds/advanced/#source-repository--branch-clones
diff --git a/hooks/build b/hooks/build
index 07a19ee..24a16d6 100644
--- a/hooks/build
+++ b/hooks/build
@@ -1,4 +1,4 @@
 #!/bin/bash
 docker build \
-        --build-arg SERVERLESS_VERSION=1.20.0 \
+        --build-arg SERVERLESS_VERSION=1.21.0 \
         -t $IMAGE_NAME .