Add MPL-2.0 header template (#87)

This template follows Mozilla's official guidance, located at
https://www.mozilla.org/en-US/MPL/headers/
diff --git a/assets/header-templates/MPL-2.0.txt b/assets/header-templates/MPL-2.0.txt
new file mode 100644
index 0000000..b0a9688
--- /dev/null
+++ b/assets/header-templates/MPL-2.0.txt
@@ -0,0 +1,3 @@
+This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0. If a copy of the MPL was not distributed with this
+file, You can obtain one at https://mozilla.org/MPL/2.0/.