[functions] Add README.md

Author: chrisbianca

functions/README.md

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+# React Firebase Hooks - Cloud Functions
+
+React Firebase Hooks provides a convenience hook for HttpsCallable functions, providing an `error` and `loading` property
+to give a complete lifecycle for executing a HttpsCallable function on Firebase Cloud Functions.
+
+All hooks can be imported from `react-firebase-hooks/functions`, e.g.
+
+```js
+import { useHttpsCallable } from 'react-firebase-hooks/functions';
+```
+
+List of Cloud Firestore hooks:
+
+- [React Firebase Hooks - Cloud Firestore](#react-firebase-hooks---cloud-firestore)
+  - [useHttpsCallable](#usehttpscallable)
+    - [Full example](#full-example)
+
+### useHttpsCallable
+
+```js
+const [executeCallable, loading, error] = useHttpsCallable(functions, name);
+```
+
+Generate a callable function and monitor its execution.
+
+The `useHttpsCallable` hook takes the following parameters:
+
+- `functions`: `functions.Functions` instance for your Firebase app
+- `name`: A `string` representing the name of the function to call
+
+Returns:
+
+- `executeCallable(data)`: a function you can call to execute the HttpsCallable
+- `loading`: a `boolean` to indicate if the function is still being executed
+- `error`: Any `Error` returned by Firebase when trying to execute the function, or `undefined` if there is no error
+
+#### Full example
+
+```js
+import { getFunctions } from 'firebase/functions';
+import { useHttpsCallable } from 'react-firebase-hooks/functions';
+
+const HttpsCallable = () => {
+  const [executeCallable, executing, error] = useHttpsCallable(
+    getFunctions(firebaseApp),
+    'myHttpsCallable'
+  );
+  return (
+    <div>
+      <p>
+        {error && <strong>Error: {JSON.stringify(error)}</strong>}
+        {executing && <span>Function executing...</span>}
+        <button
+          onClick={async () => {
+            executeCallable();
+            alert('Executed function');
+          }}
+        >
+          Execute callable function
+        </button>
+      </p>
+    </div>
+  );
+};
+```