Add migration guide for release 5.0.0 #585 (#615)

Author: kobylynskyi

docs/migration-to-5.0.0.md

@@ -0,0 +1,70 @@
+Some breaking changes were introduced
+in [Release 5.0.0](https://github.com/kobylynskyi/graphql-java-codegen/releases/tag/v5.0.0). So if you were using
+version 4.x.x then please follow steps below. Note: if you are migrating from version 3.x.x or earlier, then please also
+follow [4.0.0 migration guide](migration-to-4.0.0.md) first.
+
+## NOTE
+**Migration from 4.x.x to 5.0.0 is required only if you were using 
+[external mapping configuration](codegen-options.md#external-mapping-configuration)**. 
+Otherwise - feel free to use graphql-java-codegen-5.0.0 without any breaking changes.
+
+## Migration steps
+
+### 1. Update plugin and library versions
+
+As per plugin description: 
+[Gradle](https://github.com/kobylynskyi/graphql-java-codegen/tree/master/plugins/gradle), 
+[Maven](https://github.com/kobylynskyi/graphql-java-codegen/tree/master/plugins/maven),
+[SBT](https://github.com/kobylynskyi/graphql-java-codegen/tree/master/plugins/sbt)
+
+### 2. Rename jsonConfigurationFile => configurationFiles
+
+#### Maven
+
+```xml
+<configuration>
+    <!--OLD APPROACH-->
+    <jsonConfigurationFile>src/main/resources/mappingConfig.json</jsonConfigurationFile>
+    <!--OLD APPROACH-->
+
+    <!--NEW APPROACH-->
+    <configurationFiles>
+      <configurationFile>src/main/resources/mappingConfig.json</configurationFile>
+    </configurationFiles>
+    <!--NEW APPROACH-->
+</configuration>
+```
+
+#### Gradle
+
+```groovy
+// OLD APPROACH
+jsonConfigurationFile="src/main/resources/mappingConfig.json"
+// OLD APPROACH
+
+// NEW APPROACH
+configurationFiles=["src/main/resources/mappingConfig.json"]
+// NEW APPROACH
+```
+
+#### SBT
+
+```sbt
+// OLD APPROACH
+jsonConfigurationFile := Some("src/main/resources/mappingConfig.json")
+// OLD APPROACH
+
+// NEW APPROACH
+configurationFiles := List("src/main/resources/mappingConfig.json")
+// NEW APPROACH
+```
+
+### 3. Regenerate the code
+
+Run project build so that GraphQL classes are regenerated and your code compiles.
+
+
+---
+
+Feel free to ask any questions in [GitHub Discussions](https://github.com/kobylynskyi/graphql-java-codegen/discussions)
+or [create an issue](https://github.com/kobylynskyi/graphql-java-codegen/issues) if you discover some problems.