Add an instruction about stacktrace on x86-64 in INSTALL file. Basically, this

Add an instruction about stacktrace on x86-64 in INSTALL file. Basically, this
note was copied from google-perftools's notice (google-perftools/INSTALL).  I
modified the notice because the situation around stacktracer is a bit different
between glog and perftools.  Perftool's tcmalloc hooks malloc and its
stacktracer must not call malloc.  In glog, stacktracer with malloc can be an
issue only when users use InstallFailureSignalHandler() and a signal is raised
inside malloc.

git-svn-id: https://google-glog.googlecode.com/svn/trunk@19 eb4d4688-79bd-11dd-afb4-1d65580434c0