Added a basic README.

README.md

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+# bitcoinjs-lib
+
+A library containing Bitcoin client-side functionality in JavaScript,
+most notably ECDSA signing and verification.
+
+# Status
+
+This is currently pretty raw code. We're planning to clean it up,
+convert everything into CommonJS modules and put a flexible build
+system in place.
+
+Prototype software, use at your own peril.