mention how to implement custom validators

Author: aryaniyaps

docs/execution/index.rst

@@ -10,4 +10,4 @@ Execution
    dataloader
    fileuploading
    subscriptions
-   validation
+   queryvalidation

docs/execution/queryvalidation.rst

@@ -0,0 +1,87 @@
+Query Validation
+==========
+GraphQL uses query validators to check if Query AST is valid and can be executed. Every GraphQL server implements
+standard query validators. For example, there is an validator that tests if queried field exists on queried type, that
+makes query fail with "Cannot query field on type" error if it doesn't.
+
+To help with common use cases, graphene provides a few validation rules out of the box.
+
+
+Depth limit Validator
+-----------------
+The depth limit validator helps to prevent execution of malicious
+queries. It takes in the following arguments.
+
+- ``max_depth`` is the maximum allowed depth for any operation in a GraphQL document.
+- ``ignore`` Stops recursive depth checking based on a field name. Either a string or regexp to match the name, or a function that returns a boolean
+- ``callback`` Called each time validation runs. Receives an Object which is a map of the depths for each operation.
+
+Example
+-------
+
+Here is how you would implement depth-limiting on your schema.
+
+.. code:: python
+    from graphql import validate, parse
+    from graphene import ObjectType, Schema, String
+    from graphene.validation import depth_limit_validator
+
+
+    class MyQuery(ObjectType):
+        name = String(required=True)
+
+
+    schema = Schema(query=MyQuery)
+
+    # Queries which have a depth more than 20
+    # will not be executed.
+
+    validation_errors = validate(
+        schema=schema,
+        document_ast=parse('THE QUERY'),
+        rules=(
+            depth_limit_validator(
+                max_depth=20
+            ),
+        )
+    )
+
+
+Implementing custom validators
+------------------------------
+All custom query validators should extend the `ValidationRule <https://github.com/graphql-python/graphql-core/blob/v3.0.5/src/graphql/validation/rules/__init__.py#L37>`_
+base class importable from the graphql.validation.rules module. Query validators are visitor classes. They are
+instantiated at the time of query validation with one required argument (context: ASTValidationContext). In order to
+perform validation, your validator class should define one or more of enter_* and leave_* methods. For possible
+enter/leave items as well as details on function documentation, please see contents of the visitor module. To make
+validation fail, you should call validator's report_error method with the instance of GraphQLError describing failure
+reason. Here is an example query validator that visits field definitions in GraphQL query and fails query validation
+if any of those fields are introspection fields:
+
+.. code:: python
+    from graphql import GraphQLError
+    from graphql.language import FieldNode
+    from graphql.validation import ValidationRule
+
+
+    my_blacklist = (
+        "disallowed_field",
+    )
+
+
+    def is_blacklisted_field(field_name: str):
+        return key.lower() in my_blacklist
+
+
+    class BlackListRule(ValidationRule):
+        def enter_field(self, node: FieldNode, *_args):
+            field_name = node.name.value
+            if not is_blacklisted_field(field_name):
+                return
+
+            self.report_error(
+                GraphQLError(
+                    f"Cannot query '{field_name}': field is blacklisted.", node,
+                )
+            )
+