Add mtrace git URL to README.

05b2b73c · Silas Boyd-Wickizer · 5108276c · 05b2b73c
--- a/README
+++ b/README
 * mtrace

+  You need to build and install mtrace:
+    git+ssh://amsterdam.csail.mit.edu/home/am6/mpdev/qemu.git -b mtrace
+
  #define MTRACE 1 in param.h
  $ make mscan.out