Add explanation for clone use

Test: no code changes
Change-Id: I70c8af8261cda3163677557fd78ae0119b1f1be0
diff --git a/libmemunreachable/PtracerThread.cpp b/libmemunreachable/PtracerThread.cpp
index aca2a82..61a1d24 100644
--- a/libmemunreachable/PtracerThread.cpp
+++ b/libmemunreachable/PtracerThread.cpp
@@ -98,6 +98,7 @@
     return (*reinterpret_cast<std::function<int()>*>(arg))();
   };
 
+  // See README.md for why we create the child process this way
   child_pid_ = clone(proxy, stack_->top(), CLONE_VM | CLONE_FS | CLONE_FILES /*|CLONE_UNTRACED*/,
                      reinterpret_cast<void*>(&func_));
   if (child_pid_ < 0) {