Update usage instructions for region-locked apps (#218)

Added note about region-locked app downloads and configuration file usage.

Author: Kajaqq

USAGE-google-play.md

@@ -34,6 +34,12 @@ Likewise, a separate timezone or locale can also be specified:
 apkeep -a com.instagram.android -d google-play -o device=ad_g3_pro,locale=es_MX -e 'someone@gmail.com' -t some_aas_token .
 ```
 
+Note that if the app you're downloading is region-locked and not available in your region, the locale and timezone changes alone will not let apkeep download it. 
+In that case the download will fail with a `Invalid app response` error. 
+To download a region-locked app you first need to bind it to your account by downloading it with an IP address from a region it's available in.
+After the first download, you can download the app from any region.
+
+
 This option attempts to download a split APK if available, and falls back to the full APK:
 
 ```shell
@@ -44,6 +50,7 @@ A full list of options:
 
 * `device`: specify a device profile as described above
 * `locale`: specify a locale
+* `timezone`: specify a timezone 
 * `split_apk`: when set to `1` or `true`, attempts to download a [split APK](https://developer.android.com/studio/build/configure-apk-splits)
 * `include_additional_files`: when set to `1` or `true`, attempts to download any [additional `obb` expansion files](https://developer.android.com/google/play/expansion-files) for the app