Add new overall user guide with links to top guides (#538)

Author: jjohnstn

USER_GUIDE.md

@@ -0,0 +1,11 @@
+# Linux Tools User Guides
+* [Callgraph User Guide](CALLGRAPH_USER_GUIDE.md)
+* [Docker User Guide](DOCKER_USER_GUIDE.md)
+* [Gcov User Guide](GCOV_USER_GUIDE.md)
+* [Gproj User Guide](GPROF_USER_GUIDE.md)
+* [Libhover User Guide](LIBHOVER_USER_GUIDE.md)
+* [Perf User Guide](PERF_USER_GUIDE.md)
+* [RPM Specfile User Guide](RPMSPECFILE_USER_GUIDE.md)
+* [SystemTap User Guide](SYSTEMTAP_USER_GUIDE.md)
+* [Vagrant User Guide](VAGRANT_USER_GUIDE.md)
+* [Valgrind User Guide](VALGRIND_USER_GUIDE.md)