Adds git-superproject-update, alias to git-update-superproject

diff --git a/git-superproject-update b/git-superproject-update
new file mode 120000 (symlink)
index 0000000..ccf7d63
--- /dev/null
+++ b/git-superproject-update
@@ -0,0 +1 @@
+git-update-superproject
\ No newline at end of file