Update a migration guide with information about module exclusion

Author: fzhinkin

docs/design/0.15.0-migration-guide.md

@@ -1,14 +1,29 @@
 The major feature shipped with the release 0.15.0 of the Binary Compatibility Validator is Kotlin libraries (or KLibs) 
 ABI validation support.
 
-This support, however, comes with a few changes that break the plugin's compatibility:
+This support, however, comes with a few changes that break the plugin's compatibility or changes its behavior:
+- In multiplatform projects, empty modules are no longer ignored during ABI validation;
 - ABI file names (`api/<project name>.api`) are no longer treated in a case-insensitive way;
 - Gradle tasks provided by the plugin changed their API.
 
-If you neither configure BCV Gradle tasks manually nor have an ABI file with a name in a case different
-from how the project was named in the settings, these changes won't affect you.
+If you have a multiplatform project with modules having no sources, configure BCV Gradle tasks manually, 
+or have an ABI file with a name in a case different from how the project was named in the settings, 
+please check the following steps for smooth migration.
 
-Otherwise, the steps required for smooth migration are described below.
+### In multiplatform projects, empty modules are no longer ignored during ABI validation
+
+Previously, if there was an empty Gradle module/project (for example, test-only module), BCV ignored validation of
+such a module if the multiplatform platform plugin was applied there. However, BCV behaved differently for similar
+projects with the JVM-plugin applied.
+
+Starting from `0.15.0` behavior was aligned and now BCV no longer ignores validation of empty modules/projects.
+If you have such a module, please either generate and commit (empty) ABI dump file for it by running 
+`./gradlew apiDump`, or consider excluding the module from validation using `ignoredProjects` setting:
+```kotlin
+apiValidation {
+  ignoredProjects += "empty-module"
+}
+```
 
 ### ABI file names are no longer treated in a case-insensitive way