Add 4.0 upgrade guide (#93)

Author: morgante

CHANGELOG.md

@@ -5,7 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [4.0.0](https://www.github.com/terraform-google-modules/terraform-google-lb-http/compare/v3.2.0...v4.0.0) (2020-04-21)
-
+Please see the [upgrade guide](./docs/upgrading_to_v4.0.md) for details.
 
 ### ⚠ BREAKING CHANGES
 

docs/upgrading_to_v4.0.md

@@ -0,0 +1,58 @@
+# Upgrading to v4.0
+
+The v4.0 release contains backwards-incompatible
+changes to the backend config.
+
+## Backend Config
+`session_affinity`, `affinity_cookie_ttl_sec`, and `log_config` must now be specified
+for backends. To use the default value, specify `null`.
+
+```diff
+  module "gce-lb-http" {
+    source            = "GoogleCloudPlatform/lb-http/google"
+-   version           = "~> 3.1"
++   version           = "~> 4.0"
+
+    backends = {
+      default = {
+        description                     = null
+        protocol                        = "HTTP"
+        port                            = var.service_port
+        port_name                       = var.service_port_name
+        timeout_sec                     = 10
+        connection_draining_timeout_sec = null
+        enable_cdn                      = false
++       session_affinity                = null
++       affinity_cookie_ttl_sec         = null
++       log_config                      = null
+
+        health_check = {
+          check_interval_sec  = null
+          timeout_sec         = null
+          healthy_threshold   = null
+          unhealthy_threshold = null
+          request_path        = "/"
+          port                = var.service_port
+          host                = null
+        }
+
+        groups = [
+          {
+            # Each node pool instance group should be added to the backend.
+            group                        = var.backend
+            balancing_mode               = null
+            capacity_scaler              = null
+            description                  = null
+            max_connections              = null
+            max_connections_per_instance = null
+            max_connections_per_endpoint = null
+            max_rate                     = null
+            max_rate_per_instance        = null
+            max_rate_per_endpoint        = null
+            max_utilization              = null
+          },
+        ]
+      }
+    }
+  }
+```