Create README.md with project details

Add initial README with project description and build instructions.

Author: c6-dev

README.md

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+# webdyff
+
+A thin wrapper for [dyff](https://github.com/homeport/dyff) that builds a WebAssembly binary and provides a static webpage to run the diff, available [here](https://c6-dev.github.io/webdyff/).
+
+## Build
+
+          GOOS=js GOARCH=wasm go build -o web/app.wasm ./cmd/wasm
+          cp "$(go env GOROOT)/lib/wasm/wasm_exec.js" web/wasm_exec.js