readme: link to gem5 out of tree build question

README.adoc

@@ -4550,6 +4550,7 @@ We would like to solve that, but are blocked on either of the following:
 +
 But then we have to deal specially with the `m5` tool, which has to be cross compiled
 * gem5 out of tree build. Is there support? Looks like SCons does not have anything generic as usual and leaves the burden on project developers:
+** https://www.mail-archive.com/gem5-users@gem5.org/msg15421.html
 ** https://stackoverflow.com/questions/3720142/how-to-force-scons-output-exe-obj-lib-dll-to-specific-build-directory
 ** https://stackoverflow.com/questions/1762044/how-to-do-an-out-of-source-build-with-scons