Added a custom server example using Flask

See issue mirumee#125

docs/custom-server.rst

@@ -116,4 +116,78 @@ The following example presents a basic GraphQL server using a Django framework::
                 response["errors"] = [format_error(e) for e in result.errors]
 
             # Send response to client
-            return JsonResponse(response)
+            return JsonResponse(response)
+
+
+Basic GraphQL server with Flask
+--------------------------------
+
+The following example presents a basic GraphQL server using a Flask::
+
+    from ariadne import gql, ResolverMap, make_executable_schema
+    from ariadne.constants import PLAYGROUND_HTML
+    from graphql import format_error, graphql_sync
+    from flask import Flask, request, jsonify
+
+
+    type_defs = gql("""
+        type Query {
+            hello: String!
+        }
+    """)
+
+
+    query = ResolverMap("Query")
+
+
+    @query.field("hello")
+    def resolve_hello(_, info):
+        request = info.context
+        user_agent = request.headers.get("User-Agent", "Guest")
+        return "Hello, %s!" % user_agent
+
+
+    app = Flask(__name__)
+
+
+    @app.route('/graphql/', methods=['GET'])
+    def graphql_playgroud():
+        """Serving the GraphQL Playground
+
+        Note: This endpoint is not required if you do not want to provide the playground
+        But keep in mind that clients can still explore your API, for example by
+        using the GraphQL desktop app.
+        """
+        return PLAYGROUND_HTML, 200
+
+
+    @app.route('/graphql/', methods=['POST'])
+    def graphql_server():
+        """Serve GraphQL queries"""
+        data = request.get_json()
+        if data is None or not isinstance(data, dict):
+            return 'Bad Request', 400
+
+        variables = data.get('variables')
+        if variables and not isinstance(variables, dict):
+            return 'Bad Request', 400
+
+        # Note: Passing the request to the context is option. In Flask, the current
+        #   request is allways accessible as flask.request.
+        result = graphql_sync(
+            make_executable_schema(type_defs, query),
+            data.get('query'),
+            context_value=request,
+            variable_values=variables,
+            operation_name=data.get('operationName')
+        )
+
+        response = {"data": result.data}
+        if result.errors:
+            response["errors"] = [format_error(e) for e in result.errors]
+
+        return jsonify(response)
+
+
+    if __name__ == '__main__':
+        app.run(debug=True)