patch 8.1.0673: functionality for signs is spread out over several files

Problem:    Functionality for signs is spread out over several files.
Solution:   Move most of the sign functionality into sign.c. (Yegappan
            Lakshmanan, closes #3751)
diff --git a/src/README.txt b/src/README.txt
index e134306..6e7094c 100644
--- a/src/README.txt
+++ b/src/README.txt
@@ -35,6 +35,7 @@
 	regexp.c	pattern matching
 	screen.c	updating the windows
 	search.c	pattern searching
+	sign.c		signs
 	spell.c		spell checking
 	syntax.c	syntax and other highlighting
 	tag.c		tags