Add docs for functions (#30)

* add docs for functions

Co-authored-by: David East <[email protected]>

README.md

@@ -132,6 +132,7 @@ const url$ = getDownloadURL(ref);
 - [Authentication](docs/auth.md)
 - [Storage](docs/storage.md)
 - [Realtime Database](docs/database.md)
+- [Cloud Functions](docs/functions.md)
 
 ## Examples
 

docs/functions.md

@@ -0,0 +1,34 @@
+# RxFire Cloud Functions
+
+## Callable Functions Observables
+
+### `httpsCallable()`
+
+The `httpsCallable()` function returns an observable that calls a [callable function](https://firebase.google.com/docs/functions/callable), then emits the data returned from that function.
+
+|                 |                                                                          |
+| --------------- | ------------------------------------------------------------------------ |
+| **function**    | `httpsCallable()`                                                        |
+| **params**      | `functions: Functions`, `name: string`, `options?: HttpsCallableOptions` |
+| **import path** | `rxfire/functions`                                                       |
+| **return**      | `(data: T) => Observable<R>`                                             |
+
+#### TypeScript Example
+
+```ts
+import { httpsCallable } from "rxfire/functions";
+import { initializeApp } from "firebase/app";
+import { getFunctions } from "firebase/functions";
+
+// Set up Firebase
+const app = initializeApp({
+  /* config */
+});
+const functions = getFunctions(app);
+
+// Assume an `uppercaser` function is deployed
+const capitalizedText$ = httpsCallable<string, string>(functions, "uppercaser")("hello world");
+capitalizedText$.subscribe((text) => {
+  console.log(text);
+}); // logs "HELLO WORLD"
+```