Explain lack of source in README

Author: zdave-parity

README.md

@@ -1 +1,8 @@
-# polkajam-releases
+# PolkaJam releases
+
+You will not find any PolkaJam source code here. All PolkaJam releases are currently binary only,
+to avoid intentional or unintentional violation of the JAM prize's clean-room rule; see
+<https://jam.web3.foundation/rules>.
+
+The PolkaJam source code _will_ be released in the future, once the JAM prizes have been awarded.
+Until then, please do not ask for it.