patch 8.1.1069: source README file doesn't look nice on github

Problem:    Source README file doesn't look nice on github.
Solution:   Turn it into markdown, still readable as plain text.
            (WenxuanHuang, closes #4141)
diff --git a/Filelist b/Filelist
index c8dfe42..74b9312 100644
--- a/Filelist
+++ b/Filelist
@@ -9,7 +9,7 @@
 		appveyor.yml \
 		ci/appveyor.bat \
 		src/Make_all.mak \
-		src/README.txt \
+		src/README.md \
 		src/alloc.h \
 		src/arabic.c \
 		src/ascii.h \