Add docs

docs/en/src/SUMMARY.md

@@ -17,6 +17,7 @@
     - [Query and Mutation](query_and_mutation.md)
     - [Subscription](subscription.md)
 - [Utilities](utilities.md)
+    - [Field guard](field_guard.md)
     - [Input value validators](input_value_validators.md)
     - [Cache control](cache_control.md)
     - [Cursor connections](cursor_connections.md)

docs/en/src/field_guard.md

@@ -0,0 +1,69 @@
+# Field Guard
+
+You can define `guard` to field of an `Object`. This permit to add checks before run the code logic of the field.
+`Guard` are made of rules that you need to define before. A rule is a structure which implement the trait `Guard`.
+
+```rust
+#[derive(Eq, PartialEq, Copy, Clone)]
+enum Role {
+    Admin,
+    Guest,
+}
+
+struct RoleGuard {
+    role: Role,
+}
+
+#[async_trait::async_trait]
+impl Guard for RoleGuard {
+    async fn check(&self, ctx: &Context<'_>) -> Result<()> {
+        if ctx.data_opt::<Role>() == Some(&self.role) {
+            Ok(())
+        } else {
+            Err("Forbidden".into())
+        }
+    }
+}
+```
+
+Once you have defined your rule you can use it in the `guard` field attribute.
+This attribute support 4 operators to create complex rules :
+
+- `and` : perform a `and` operation between two rules. (If one rule return an error the `and` operator will return the error. If both rules return a error it's the first one that will be returned).
+
+- `or` : perform a `and` operation between two rules. (If both rules return an error the error returned is the first one)
+
+- `chain` : take a set of rules and run them until one return an error or return `Ok()` if all rules pass.
+
+- `race` : take a set of rules and run them until one return `Ok()` if they all fail it return the last error.
+
+```rust
+#[derive(SimpleObject)]
+struct Query {
+    #[graphql(guard(RoleGuard(role = "Role::Admin")))]
+    value: i32,
+    #[graphql(guard(and(
+        RoleGuard(role = "Role::Admin"),
+        UserGuard(username = r#""test""#)
+    )))]
+    value2: i32,
+    #[graphql(guard(or(
+        RoleGuard(role = "Role::Admin"),
+        UserGuard(username = r#""test""#)
+    )))]
+    value3: i32,
+    #[graphql(guard(chain(
+        RoleGuard(role = "Role::Admin"),
+        UserGuard(username = r#""test""#),
+        AgeGuard(age = r#"21"#)
+    )))]
+    value4: i32,
+    #[graphql(guard(race(
+        RoleGuard(role = "Role::Admin"),
+        UserGuard(username = r#""test""#),
+        AgeGuard(age = r#"21"#)
+    )))]
+    value5: i32,
+}
+```
+