Add updte-gh-pages script

pkgs/markdown/tool/update-gh-pages.sh

diff --git a/pkgs/markdown/tool/update-gh-pages.sh b/pkgs/markdown/tool/update-gh-pages.sh
new file mode 100755
index 0000000..64962a7
--- /dev/null
+++ b/pkgs/markdown/tool/update-gh-pages.sh
@@ -0,0 +1,13 @@
+# Echo every command being run.
+set +x
+
+# Fail fast if a command fails.
+set -e
+
+pub global activate peanut
+
+peanut -d example
+
+echo Now push updated gh-pages branch with:
+echo
+echo '    git push origin --set-upstream gh-pages'