Add a basic tutorial for the tools in depot_tools.

docs/src/git-freeze.demo.1.sh

Index: docs/src/git-freeze.demo.1.sh
diff --git a/docs/src/git-freeze.demo.1.sh b/docs/src/git-freeze.demo.1.sh
new file mode 100755
index 0000000000000000000000000000000000000000..13db046dd23e0c9b49edf9c8be78e0e14ef1003a
--- /dev/null
+++ b/docs/src/git-freeze.demo.1.sh
@@ -0,0 +1,23 @@
+#!/bin/sh
+. demo_repo.sh
+
+add deleted_file
+add unstaged_deleted_file
+add modified_file
+c "demo changes"
+
+add added_file_with_unstaged_changes
+echo bob >> added_file_with_unstaged_changes
+
+add added_file
+echo bob >> modified_file
+silent git rm deleted_file
+rm unstaged_deleted_file
+touch unadded_file
+
+run git status --short
+run git freeze
+run git status --short
+run git log -n 2 --stat
+run git thaw
+run git status --short