add docs about swift-java.config

Author: ktoso

Sources/SwiftJavaDocumentation/Documentation.docc/SwiftJavaCommandLineTool.md

@@ -254,3 +254,14 @@ public final class SomeModule ... {
     public static void globalFunction() { ... }
 }
 ```
+
+### The swift-java.config file
+
+Many of the tools–as well as SwiftPM plugin's–behaviors can be configured using the `swift-java.config` file.
+
+You can refer to the ``SwiftJavaSharedConfiguration/Configuration`` struct to learn about the supported options.
+
+Configuration from the config files may be overriden or augmented by explicit command line parameters,
+please refer to the options documentation for details on their behavior.
+
+> Note: **Comments in configuration**: The configuration is a JSON 5 file, which among other things allows `//` and `/* */` comments, so feel free to add line comments explaining rationale for some of the settings in youf configuration.