docs: add MPL-2.0 Exhibit A header and dual attribution

Add the standard MPL Exhibit A notice at the top of init.lua along
with explicit copyright lines for both the original author (Yorick
Peterse) and the modifications in this fork. The LICENSE file
already carries the full MPL-2.0 text; this just makes per-file
provenance unambiguous.

lua/tiny-jump/init.lua

@@ -1,3 +1,11 @@
+-- tiny-jump.nvim — a fork of https://github.com/yorickpeterse/nvim-jump
+-- Original work: Copyright (c) Yorick Peterse
+-- Modifications: Copyright (c) 2026 Stephen Waits
+--
+-- 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 http://mozilla.org/MPL/2.0/.
+
 local api = vim.api
 local fn = vim.fn
 local M = {}