docs: 仓库首页改用自述说明；原作者 README 存档为 README.UPSTREAM.md；LICENSE 追加修改者声明

LICENSE

@@ -19,3 +19,8 @@ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
+
+---
+
+本仓库包含对上述项目的修改（修改清单见 README.CUSTOM.md）。
+Modifications Copyright (c) 2026 MuLin