Document specifiedBy directive handling

Author: wravery

doc/directives.md

@@ -2,7 +2,7 @@
 
 Directives in GraphQL are extensible annotations which alter the runtime
 evaluation of a query or which add information to the `schema` definition.
-They always begin with an `@`. There are three built-in directives which this
+They always begin with an `@`. There are four built-in directives which this
 library automatically handles:
 
 1. `@include(if: Boolean!)`: Only resolve this field and include it in the
@@ -11,9 +11,11 @@ results if the `if` argument evaluates to `true`.
 results if the `if` argument evaluates to `false`.
 3. `@deprecated(reason: String)`: Mark the field or enum value as deprecated
 through introspection with the specified `reason` string.
+4. `@specifiedBy(url: String!)`: Mark the custom scalar type through
+introspection as specified by a human readable page at the specified URL.
 
 The `schema` can also define custom `directives` which are valid on different
 elements of the `query`. The library does not handle them automatically, but it
-will pass them to the `getField` implementations through the
+will pass them to the `getField` implementations through the optional
 `graphql::service::FieldParams` struct (see [fieldparams.md](fieldparams.md)
 for more information).