docs: Add command line to generate tarball with a prefix

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/README.tarball b/README.tarball
new file mode 100644
index 0000000..a3db289
--- /dev/null
+++ b/README.tarball
@@ -0,0 +1 @@
+tar cvfJ ~/rpmbuild/SOURCES/dwarves-1.17.tar.xz --transform 's,^,dwarves-1.17/,' `cat MANIFEST`