add jitpack in README

@@ -17,6 +17,19 @@ dependencies {
 }
 ```
 
+You'll also need jitpack as one of your repositories.
+
+```groovy
+subprojects {
+    repositories {
+        google()
+        mavenCentral()
+        // ...
+        maven { url 'https://jitpack.io' }
+    }
+}
+```
+
 ## Sample App
 
 There are two sample apps with similar functionality: