Create api-gateway.md

docs/aws/api-gateway.md

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+# API Gateway
+
+## Lambda Authorizers
+
+Lambda Authorizers can be configured with Identity Sources, which are effectively mandatory inputs. They can be query string params, HTTP request headers, etc. API Gateway will reject requests with a 401 response if a request does not contain the mandatory Identity Sources. To use a lambda authorizer which uses one or another method of authentication, specify a blank list for the Identity Sources instead. 
+
+See <https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-use-lambda-authorizer.html>