Clarify where to apply the android plugin

This is in response to this comment Kotlin#2227 (comment)
We spent a fair amount of time trying to figure out why it wasn't working for us & putting this extra line
in the documentation would've saved us a lot of time.

README.md

@@ -79,6 +79,10 @@ apply(plugin= "org.jetbrains.dokka")
 ```
 
 ```kotlin
+dependencies {
+    dokkaHtmlPlugin("org.jetbrains.dokka:android-documentation-plugin:${dokka_version}")
+}
+
 dokkaHtml.configure {
     dokkaSourceSets {
         named("main") {
@@ -88,6 +92,9 @@ dokkaHtml.configure {
 }
 ```
 
+**NOTE:** replace `Html` in the above code with whatever [output format](#output_formats) you need. 
+
+
 #### Multi-module projects
 For documenting Gradle multi-module projects, you can use `dokka${format}Multimodule` tasks.