Added documenation on using chmod. Added Windodws support by making t…

…he call a noop.

lib/src/functions/chmod.dart

@@ -1,3 +1,5 @@
+import 'dart:io';
+
 import '../../dcli.dart';
 import '../util/dcli_exception.dart';
 import '../util/stack_trace_impl.dart';
@@ -9,6 +11,29 @@ import 'dcli_function.dart';
 /// [permission] is the standard bit map used by chmod e.g. 777
 /// [path] is the path to the file that we are changing the
 /// permissions of.
+///
+/// The the [permission] digits are intrepeted as owner, group, other.
+/// So:
+/// 641
+/// owner - 6
+/// group - 4
+/// other - 1
+///
+/// Each digit is the sum of the permissions:
+/// 4 - allow read
+/// 2 - allow write
+/// 1 - all execute
+///
+/// So 6 is 4 + 2 is read and write.
+///
+/// To set give the owner execution privileges use:
+/// ```dart
+/// chmod(100, '/path/to/exe');
+/// ```
+/// If [path] doesn't exist a ChModException] is thrown.
+///
+/// On Windows a call to this method is a noop.
+///
 void chmod(int permission, String path) => _ChMod()._chmod(permission, path);
 
 /// Implementatio for [chmod] function.
@@ -19,7 +44,9 @@ class _ChMod extends DCliFunction {
     if (!exists(path)) {
       throw ChModException('The file at ${truepath(path)} does not exists');
     }
-    'chmod $permission "$path"'.run;
+    if (!Platform.isWindows) {
+      'chmod $permission "$path"'.run;
+    }
   }
 
   //  String chmod({int user, int group, int other, this.path}) {}