add mirror config doc (#2917)

Add mirror config file documentation.

Pull request: #2917

---------
Co-authored-by: Tobias Roeser <le.petit.fou@web.de>

example/scalamodule/11-repository-config/build.sc

@@ -17,12 +17,30 @@ object foo extends ScalaModule {
   }
 }
 
+// Mill read https://get-coursier.io/[coursier] config files automatically.
+//
+// It is possible to setup mirror with `mirror.properties`
+// [source,properties]
+// ----
+// central.from=https://repo1.maven.org/maven2
+// central.to=http://example.com:8080/nexus/content/groups/public
+// ----
+//
+// Note theses default config file locatations:
+//
+// * Linux: `~/.config/coursier/mirror.properties`
+// * MacOS: `~/Library/Preferences/Coursier/mirror.properties`
+// * Windows: `C:\Users\<user_name>\AppData\Roaming\Coursier\config\mirror.properties`
+//
+// You can also set the environment variable `COURSIER_MIRRORS` or the jvm property `coursier.mirrors` to specify config file location.
+
+
 // To add custom resolvers to the initial bootstrap of the build, you can create a
 // custom `ZincWorkerModule`, and override the `zincWorker` method in your
 // `ScalaModule` by pointing it to that custom object:
 
 object CustomZincWorkerModule extends ZincWorkerModule with CoursierModule {
-  def repositoriesTask() = T.task { super.repositoriesTask() ++ sonatypeReleases }
+  def repositoriesTask = T.task { super.repositoriesTask() ++ sonatypeReleases }
 }
 
 object bar extends ScalaModule {
@@ -37,4 +55,4 @@ object bar extends ScalaModule {
 
 > ./mill bar.compile
 
-*/
+*/