Added graphql @Bind doc

Author: Aklakan

docs/graphql/reference/bind.md

@@ -0,0 +1,38 @@
+---
+title: bind
+parent: GraphQL
+nav_order: 130
+layout: default
+---
+
+## GraphQL Directive: `@bind`
+
+The `@bind` directive is used to associate a field in your GraphQL query or schema with a SPARQL expression. Note that [`@pattern`](pattern) is for graph patterns rather than expressions.
+As bind only produces a single value from the input binding, its cardinality defaults to `@one`. Explicitly adding `@many` will cause the value to be wrapped in an array.
+
+### Purpose
+
+The directive allows one to compute values for a field based on variables that are mentioned in any ancestor node. The most common use is to expose an entity's IRI (or blank node) as an `id` field.
+
+#### Usage
+
+The `@bind` directive supports the following arguments:
+
+- **`of`** (`String`): The value of `of` is a SPARQL expression. The mentioned variables must be defined in any ancestor or at the annotated field.
+- **`as`** (`String`): (optional) The variable for the expression result can be specified explicitly (by default an internal name is generated).
+- **`target`** (`Boolean`): (optional, defaults to `true`) Whether the `@bind` expression produces the target values for the annotated field.
+    If `target` is `false` then the field introduces the specified variable that can be referenced from descendants without it becoming a target of the field.
+    Multiple target variables are combined into a tuple based on the order of their appearance.
+
+#### Example
+
+Here is an example demonstrating how to define a `MusicalArtists` field using the `@bind` directive:
+
+```graphql
+{
+  MusicalArtists @pattern(of: "?s a <http://dbpedia.org/ontology/MusicalArtist>") {
+    id @bind(of: "?s")
+  }
+}
+```
+