feat: add docs for functions-ktx (#506)

Author: thatfiredev

docs/README.md

@@ -25,6 +25,7 @@ in your app:
 
   * [`firebase-common`](ktx/common.md)
   * [`firebase-firestore`](ktx/firestore.md)
+  * [`firebase-functions`](ktx/functions.md)
 
 [android-setup]: https://firebase.google.com/docs/android/setup
 [main-readme]: https://github.com/firebase/firebase-android-sdk/blob/master/README.md

docs/ktx/functions.md

@@ -0,0 +1,62 @@
+# Cloud Functions Kotlin Extensions
+
+## Getting Started
+
+To use the Cloud Functions Android SDK with Kotlin Extensions, add the following
+to your app's `build.gradle` file:
+
+```groovy
+// See maven.google.com for the latest versions
+// This library transitively includes the firebase-functions library
+implementation 'com.google.firebase:firebase-functions-ktx:$VERSION'
+```
+
+## Features
+
+### Get the FirebaseFunctions instance of the default app
+
+**Kotlin**
+```kotlin
+val functions = FirebaseFunctions.getInstance()
+```
+
+**Kotlin + KTX**
+```kotlin
+val functions = Firebase.functions
+```
+
+### Get the FirebaseFunctions of a given region
+
+**Kotlin**
+```kotlin
+val functions = FirebaseFunctions.getInstance(region)
+```
+
+**Kotlin + KTX**
+```kotlin
+val functions = Firebase.functions(region)
+```
+
+### Get the FirebaseFunctions of a given FirebaseApp
+
+**Kotlin**
+```kotlin
+val functions = FirebaseFunctions.getInstance(app)
+```
+
+**Kotlin + KTX**
+```kotlin
+val functions = Firebase.functions(app)
+```
+
+### Get the FirebaseFunctions of a given region and FirebaseApp
+
+**Kotlin**
+```kotlin
+val functions = FirebaseFunctions.getInstance(app, region)
+```
+
+**Kotlin + KTX**
+```kotlin
+val functions = Firebase.functions(app, region)
+```