Add a README

Author: TheBlueMatt

README

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+This crate contains a basic parser for Bitcoin Payment Instructions.
+
+It supports various encodings which you might encounter in QR codes,
+   typed/pasted into a text box, or opened via a system URI handler.
+
+Any missing Payment Instruction formats should be added as soon as there is a
+standard encoding for address formats, ideally using a Rust crate that (a) has
+very minimal dependencies, (b) has a fairly conservative MSRV policy, and
+(c) has regular contribution from more than one individual.