diff --git a/sdk_build/README.md b/sdk_build/README.md
index f8cc65a..4cd0a5e 100644
--- a/sdk_build/README.md
+++ b/sdk_build/README.md
@@ -2,3 +2,19 @@
 
 This directory contains (or will contain) tools, scripts, and data for building
 SDKs (for various languages and platforms).
+
+## build_sdk.py
+
+This is a script that creates a directory containing an "SDK" using the
+instructions in a given "SDK specification file" and data from the current git
+repository (which should be clean; for testing purposes, you may give the
+`--allow-dirty-tree` flag).
+
+For example:
+
+    $ ./build_sdk.py data/cpp/cpp.sdk /tmp/my_cpp_sdk
+
+This creates an SDK for C++ in `/tmp/my_cpp_sdk`.
+
+This script does not handle packaging such an SDK (e.g., into a tarball) or
+uploading it (e.g., to Google Cloud Storage).