patch 8.0.0509: no link to codecov.io results

Problem:    No link to codecov.io results.
Solution:   Add a badge to the readme file.
diff --git a/src/version.c b/src/version.c
index 48f0a4c..cb02f47 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    509,
+/**/
     508,
 /**/
     507,