DEV-3385 docs(onlyf):add subject

subjects/onlyf/README.md

@@ -0,0 +1,16 @@
+## onlyf
+
+### Instructions
+
+Write a program that displays an `f` character on the standard output. (and nothing else)
+
+### Usage
+
+```console
+$ go run .
+a
+$ go run . "a" "b"
+a
+$ go run . "a" "b" "c"
+a
+```