Add README describing current state.

README.md

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+TextSecure Chromium Implementation
+==================================
+
+This is very early stuff and exists primarily to get the crypto in place.
+It is currently chromium-only as it uses NaCL for Curve25519 stuff, but I'd be
+glad to accept a pull that abstracts out the NaCL-specific stuff to optionally
+use a JS implementation for FF.
+Note that the code is currently quite messy (its all in one file!), but it
+needs to work first, then it can be heavily cleaned up later.