Add README and LICENSE

README.md

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+# Overview
+
+This is a simple library for parsing DER-encoded data.
+
+In particular, this library automates the process of understanding the DER
+encoded objects in an ASN.1 data stream. These tokens can then be parsed by your
+library, based on the ASN.1 description in your format.
+
+For convenience, we create the traits `ToASN1` and `FromASN` to abstract the
+ability to decode a type from an ASN.1 token stream. If your type implements one
+of these traits, your program or library can then use the convenience functions
+`der_encode` and `der_decode` to do all the parsing work in one action.
+
+Patches welcome!
+