Don't provide 'api_id' and 'api_hash' by default.

We ask the developer to obtain his own api credentials, because
the bundled 'api_id' / 'api_hash' are strictly limited by the server.

The old credentials still could be used for test purposes,
but the developer will need to explicitly opt-in to use them.

docs/building-xcode.md

@@ -4,6 +4,10 @@
 
 Choose a folder for the future build, for example **/Users/user/TBuild**. It will be named ***BuildPath*** in the rest of this document. All commands will be launched from Terminal.
 
+### Obtain your API credentials
+
+You will require **api_id** and **api_hash** to access the Telegram API servers. To learn how to obtain them [click here][api_credentials].
+
 ### Download libraries
 
 Download [**xz-5.0.5**](http://tukaani.org/xz/xz-5.0.5.tar.gz) and unpack to ***BuildPath*/Libraries/xz-5.0.5**
@@ -138,3 +142,5 @@ Go to ***BuildPath*/tdesktop/Telegram** and run
     gyp/refresh.sh
 
 Then launch Xcode, open ***BuildPath*/tdesktop/Telegram/Telegram.xcodeproj** and build for Debug / Release.
+
+[api_credentials]: api_credentials.md