Add missing license file

Signed-off-by: Seonghyun Kim <[email protected]>

LICENSE.BOEHM-GC

@@ -0,0 +1,30 @@
+Copyright (c) 1988, 1989 Hans-J. Boehm, Alan J. Demers
+Copyright (c) 1991-1996 by Xerox Corporation.  All rights reserved.
+Copyright (c) 1996-1999 by Silicon Graphics.  All rights reserved.
+Copyright (c) 1999-2004 Hewlett-Packard Development Company, L.P.
+
+The file linux_threads.c is also
+Copyright (c) 1998 by Fergus Henderson.  All rights reserved.
+
+The files Makefile.am, and configure.in are
+Copyright (c) 2001 by Red Hat Inc. All rights reserved.
+
+Several files supporting GNU-style builds are copyrighted by the Free
+Software Foundation, and carry a different license from that given
+below.
+
+THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
+OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
+
+Permission is hereby granted to use or copy this program
+for any purpose,  provided the above notices are retained on all copies.
+Permission to modify the code and to distribute modified code is granted,
+provided the above notices are retained, and a notice that the code was
+modified is included with the above copyright notice.
+
+A few of the files needed to use the GNU-style build procedure come with
+slightly different licenses, though they are all similar in spirit.  A few
+are GPL'ed, but with an exception that should cover all uses in the
+collector.  (If you are concerned about such things, I recommend you look
+at the notice in config.guess or ltmain.sh.)
+