decodetree: Move documentation to docs/devel/decodetree.rst

One great big block comment isn't the best way to document
the syntax of a language.

Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 3426d33..564b8db 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -118,6 +118,7 @@
 F: accel/tcg/
 F: accel/stubs/tcg-stub.c
 F: scripts/decodetree.py
+F: docs/devel/decodetree.rst
 F: include/exec/cpu*.h
 F: include/exec/exec-all.h
 F: include/exec/helper*.h