tcg/README: document tcg_gen_goto_tb restrictions

See
http://lists.nongnu.org/archive/html/qemu-devel/2012-09/msg03196.html
for the whole story.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

tcg/README

diff --git a/tcg/README b/tcg/README
index d03ae05..33783ee 100644
--- a/tcg/README
+++ b/tcg/README
@@ -392,7 +392,8 @@
 
 Exit the current TB and jump to the TB index 'index' (constant) if the
 current TB was linked to this TB. Otherwise execute the next
-instructions.
+instructions. Only indices 0 and 1 are valid and tcg_gen_goto_tb may be issued
+at most once with each slot index per TB.
 
 * qemu_ld8u t0, t1, flags
 qemu_ld8s t0, t1, flags