Add clarification (based on #212)

Change-Id: Id60b0b3ed25d44f3e3da027427a4337dd6672e92

Author: samtstern

README.md

@@ -128,6 +128,12 @@ these permissions from the user and they must be changed in app settings. You ca
 method `EasyPermissions.somePermissionPermanentlyDenied(...)` to display a dialog to the
 user in this situation and direct them to the system setting screen for your app:
 
+**Note**: Due to a limitation in the information provided by the Android
+framework permissions API, the `somePermissionPermanentlyDenied` method only
+works after the permission has been denied and your app has received
+the `onPermissionsDenied` callback. Otherwise the library cannot distinguish
+permanent denial from the "not yet denied" case.
+
 ```java
 @Override
 public void onPermissionsDenied(int requestCode, List<String> perms) {

easypermissions/src/main/java/pub/devrel/easypermissions/EasyPermissions.java

@@ -291,6 +291,12 @@ public static void onRequestPermissionsResult(int requestCode,
      * Check if at least one permission in the list of denied permissions has been permanently
      * denied (user clicked "Never ask again").
      *
+     * <b>Note</b>: Due to a limitation in the information provided by the Android
+     * framework permissions API, this method only works after the permission
+     * has been denied and your app has received the onPermissionsDenied callback.
+     * Otherwise the library cannot distinguish permanent denial from the
+     * "not yet denied" case.
+     *
      * @param host              context requesting permissions.
      * @param deniedPermissions list of denied permissions, usually from {@link
      *                          PermissionCallbacks#onPermissionsDenied(int, List)}