readme.txt: direct users to the other branches.

Add a note in the readme for new users explaining there are multiple
branches with different driver implementations.

readme.txt

@@ -1,3 +1,13 @@
+Note:
+
+This repository has multiple branches with different implementations for
+Realtek bluetooth usb drivers. Depending on your kernel version and your
+hardware model different branches may or may not work. If you are unsure
+which branch to use you and you are on a modern kernel you should probably
+start with the 'kernel' branch. If that doesn't work you are encouraged
+to try the other branches and to check the github issues.
+
+===========================================================================================================
 
 In this document, we introduce how to support rtk 8723AE/AU BT driver in Linux system.
 Support kernel version 2.6.32~3.13.0