repl_www: add a README with diagram

repl_www/README.md

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+# Web REPL
+
+## High-level diagram
+
+![High-level diagram](./repl-www.png)
+
+## Steps
+
+- User types text into the HTML `<input />` tag
+- JS detects the `onchange` event and passes the input text to the Roc compiler WebAssembly module
+- Roc compiler WebAssembly module
+    - Parses the text (currently just a single line)
+    - Type checks
+    - Monomorphizes
+    - Generates WebAssembly using the development backend (not LLVM)
+    - Returns a slice of bytes to JavaScript
+- JavaScript
+    - Takes the slice of bytes and creates a `WebAssembly.Instance`
+    - Runs the WebAssembly app
+    - Gets the memory address of the result and makes a copy of the app's entire memory buffer
+    - Passes the result address and the memory buffer to the compiler for analysis
+- Roc compiler WebAssembly module
+    - Analyses the bytes of the result, based on the known return type from earlier
+    - Traverses the copied memory buffer to find any child values
+    - Produces a user-friendly String and passes it to JavaScript
+- JavaScript
+    - Displays the input and output text on the web page
+
+## Related crates
+
+There are several directories/packages involved here:
+- `repl_www`: The web page with its JavaScript and a build script
+- `repl_wasm`: The Rust crate that becomes the "compiler" WebAssembly module
+- `repl_eval`: REPL logic shared between `repl_cli` and `repl_wasm`
+