Document route breaking change.

docs/en/Migration-Guides/Abp-4_0.md

@@ -1,5 +1,42 @@
 # ABP Framework 3.3 to 4.0 Migration Guide
 
+## Auto API Controller Route Changes
+
+The route calculation for the [Auto API Controllers](https://docs.abp.io/en/abp/latest/API/Auto-API-Controllers) is changing with the ABP Framework version 4.0 ([#5325](https://github.com/abpframework/abp/issues/5325)). Previously, **camelCase** route paths were being used. Beginning from the version 4.0, it uses **kebab-case** route paths where it is possible.
+
+**A typical auto API before v4.0**
+
+![route-before-4](images/route-before-4.png)
+
+**camelCase route parts become kebab-case with 4.0**
+
+![route-4](images/route-4.png)
+
+If it is hard to change it for your application, you can continue to use the version 3.x route strategy, by following one of the approaches;
+
+* Set `UseV3UrlStyle` to `true` in the options of the `options.ConventionalControllers.Create(...)` method. Example:
+
+````csharp
+options.ConventionalControllers
+    .Create(typeof(BookStoreApplicationModule).Assembly, opts =>
+        {
+            opts.UseV3UrlStyle = true;
+        });
+````
+
+This approach effects only the controllers for the `BookStoreApplicationModule`.
+
+* Set `UseV3UrlStyle` to `true` for the `AbpConventionalControllerOptions` to set it globally. Example:
+
+```csharp
+Configure<AbpConventionalControllerOptions>(options =>
+{
+    options.UseV3UrlStyle = true;
+});
+```
+
+Setting it globally effects all the modules in a modular application.
+
 ## Identity Server Changes
 
 ABP Framework upgrades the [IdentityServer4](https://www.nuget.org/packages/IdentityServer4) library from 3.x to 4.x with the ABP Framework version 4.0. IdentityServer 4.x has a lot of changes, some of them are **breaking changes in the data structure**.