Basic readme

README.md

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+# Diffenator3
+
+This is a Rust port of
+[diffenator2](https://github.com/googlefonts/diffenator2), a utility for
+comparing two font files. It comes in two flavours, command line and
+WASM.
+
+The command line version is still in very early development, and doesn't
+do very much at the moment; it compares two fonts (with burned-in path
+names) and outputs a JSON dictionary showing the differences between
+them. 
+
+The WASM version is a bit more interesting; it can take two font files
+over the web and display a HTML report similar to diffenator2. It
+doesn't yet compare variable fonts in the same way.
+
+You can use the WASM version at https://simoncozens.github.io/diffenator3
+
+## To rebuild the WASM version
+
+* cargo build --target wasm32-unknown-unknown 
+* cd www; npm run build