Add the beginnings of scripts to produce SDKs.

R=vardhan@google.com

Review URL: https://codereview.chromium.org/1701323003 .
diff --git a/sdk_build/README.md b/sdk_build/README.md
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+# Tools/scripts/data for building SDKs
+
+This directory contains (or will contain) tools, scripts, and data for building
+SDKs (for various languages and platforms).