add nth/3 description to README

README.md

@@ -222,6 +222,7 @@ for sols.Next() {
 | List Processing      | `append(List1, List2, List3)`                    |      | Succeeds if `List3` is the concatination of `List1` and `List2`.                                                                                                                                                | Prolog                                                                                   |
 |                      | `member(Elem, List)`                             |      | Succeeds if `Elem` is a member of `List`.                                                                                                                                                                       | Prolog                                                                                   |
 |                      | `length(List, Length)`                           |      | Succeeds if `Length` is the length of `List`.                                                                                                                                                                   | Prolog                                                                                   |
+|                      | `nth(N, List, Elem)`                             |      | Succeeds if `Elem` is the `N`-th element of `List`.                                                                                                                                                             | Prolog                                                                                   |
 | Term Expansion       | `expand_term(In, Out)`                           |      | Unifies `Out` with an expanded term for `In`.                                                                                                                                                                   | [Go](https://pkg.go.dev/github.com/ichiban/prolog/engine#State.ExpandTerm)               |
 | Environment Variable | `environ(Key, Value)`                            |      | Succeeds if an environment varialble `Key` has a value `Value`.                                                                                                                                                 | [Go](https://pkg.go.dev/github.com/ichiban/prolog/engine#Environ)                        |