Docs: Add usage documentation to README

README.md

@@ -14,11 +14,15 @@
 
 ## Description
 
-TODO
+Detect when and where mutations occur in your code. It may seem cool, but it's not a replacement for strict typings or tests. It is currently a proof-of-concept and a debugging tool.
+
+> Why did this property change?! I didn't do that!!!
 
 ## Features
 
-- TODO
+- Detect any proxy-compatible mutation as it happens
+- Throws a stack trace of the code that caused the mutation
+- Includes a property path to the mutation in the object
 
 ## Installation
 
@@ -28,7 +32,48 @@ npm install will-mutate @babel/core@^7.0.0 --save-dev
 
 ## Usage
 
-TODO
+Add the `$shouldNotMutate` function call directly above any function declaration or expression with the name of any variable in-scope at the beginning of the function body.
+
+### Code
+
+```js
+// In this example, we're asserting that the argument `foo` will not mutate
+$shouldNotMutate(["foo"]);
+function bar (foo, other) {
+    foo.prop = "Test";
+    other.prop = "Don't change me";
+}
+```
+
+#### Errors
+
+The below error is taken from running the above code snippet with `node ./bar.js` after compilation with Babel. You can see that the `prop` property was changed using `set` (assignment operator) inside the `bar` function in the `bar.js` module.
+
+
+```bash
+Error: Mutation assertion failed. `set` trap triggered on `target.prop`.
+    at Object.handler.<computed> [as set] (bar.js:148:13)
+    at bar (bar.js:194:31)
+    at Object.<anonymous> (bar.js:211:1)
+```
+
+If you see a trap you do not understand, [MDN has a list of proxy traps](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy#Handler_functions) where you can understand the conditions it will trigger under. Every trap related to mutations is implemented.
+
+### Babel Config
+
+You must also add Will Mutate to your Babel config. If you intend to leave the functions in your codebase you can use `noop` to run whenever you do not want runtime errors (e.g. production).
+
+`babel.config.js`
+
+```js
+module.exports = {
+    plugins: [
+        process.env.NODE_ENV === "development"
+            ? "will-mutate"
+            : "will-mutate/noop",
+    ],
+};
+```
 
 ## License