docs: add new rule docs

Author: gajus

.README/README.md

@@ -154,7 +154,7 @@ When `true`, only checks files with a [`@flow` annotation](http://flowtype.org/d
 {"gitdown": "include", "file": "./rules/define-flow-type.md"}
 {"gitdown": "include", "file": "./rules/delimiter-dangle.md"}
 {"gitdown": "include", "file": "./rules/generic-spacing.md"}
-{"gitdown": "include", "file": "./rules/newline-after-flow-annotation"}
+{"gitdown": "include", "file": "./rules/newline-after-flow-annotation.md"}
 {"gitdown": "include", "file": "./rules/no-dupe-keys.md"}
 {"gitdown": "include", "file": "./rules/no-existential-type.md"}
 {"gitdown": "include", "file": "./rules/no-flow-fix-me-comments.md"}