patch 8.1.0483: MinGW does not build tee.exe

Problem:    MinGW does not build tee.exe.
Solution:   Add build instructions. (Yasuhiro Matsumoto, closes #3548)
diff --git a/src/tee/Makefile b/src/tee/Makefile
index 951a1cc..12c43d5 100644
--- a/src/tee/Makefile
+++ b/src/tee/Makefile
@@ -1,4 +1,4 @@
-# A very (if not the most) simplistic Makefile for OS/2
+# A very (if not the most) simplistic Makefile for MS-Windows and OS/2
 
 CC=gcc
 CFLAGS=-O2 -fno-strength-reduce