Write directives.md content

Author: wravery

doc/directives.md

@@ -0,0 +1,19 @@
+# Directives
+
+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
+library automatically handles:
+
+1. `@include(if: Boolean!)`: Only resolve this field and include it in the
+results if the `if` argument evaluates to `true`.
+2. `@skip(if: Boolean!)`: Only resolve this field and include it in the
+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.
+
+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
+`graphql::service::FieldParams` struct (see [fieldparams.md](fieldparams.md)
+for more information).