Adds some API doc

Author: amlcurran

sample/src/main/java/com/github/amlcurran/showcaseview/sample/ViewTargets.java

@@ -7,8 +7,17 @@
 
 import java.lang.reflect.Field;
 
+/**
+ * A collection of not-officially supported ViewTargets. Use them at your own risk!
+ */
 public class ViewTargets {
 
+    /**
+     * Highlight the navigation button (the Up or Navigation drawer button) in a Toolbar
+     * @param toolbar The toolbar to search for the view in
+     * @return the {@link ViewTarget} to supply to a {@link com.github.amlcurran.showcaseview.ShowcaseView}
+     * @throws MissingViewException when the view couldn't be found. Raise an issue on Github if you get this!
+     */
     public static ViewTarget navigationButtonViewTarget(Toolbar toolbar) throws MissingViewException {
         try {
             Field field = Toolbar.class.getDeclaredField("mNavButtonView");