Fix sdbus++-gen-meson version warning

WARNING: Generated meson files from wrong version of
sdbus++-gen-meson.
WARNING: Expected "sdbus++-gen-meson version 1",
got: sdbus++-gen-meson version 3

Add the regenerate-meson file to automatically update meson.build.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: If1ce533bc0f6bafbe75d05595f9d36f5f652bd45
diff --git a/gen/README b/gen/README
new file mode 100644
index 0000000..71182b5
--- /dev/null
+++ b/gen/README
@@ -0,0 +1,2 @@
+This directory contains generated meson.build files from sdbus++-gen-meson.
+Do not edit them by hand.