packet.h clarification (#2639)

* Added comments to clarify PACKETVER changes.
Thanks to @sader1992!

src/config/packets.h

@@ -9,6 +9,9 @@
  **/
 
 #ifndef PACKETVER
+	/// Do NOT edit this line! To set your client version, please do this instead:
+	/// In Windows: Add this line in your src\custom\defines_pre.h file: #define PACKETVER YYYYMMDD
+	/// In Linux: The same as above or run the following command: ./configure --enable-packetver=YYYYMMDD
 	#define PACKETVER 20151104
 #endif