Add mDNS discovery to the Fletch compiler

pkg/fletchc/lib/src/verbs/documentation.dart

 // Copyright (c) 2015, the Fletch project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 library fletchc.verbs.documentation;
 
 const String synopsis = """
 Manages interactions with the fletch compiler and runtime.
 Example: fletch run sample.dart
 
@@ skipping 142 matching lines @@
    show log [in session <name>]
              Display log for given session
 """;
 
 // TODO(lukechurch): Review UX.
 const String upgradeDocumentation = """
    x-upgrade agent with <package-file> [in session <session>]
              Upgrade the agent used in session to the version provided in the
              .deb package <package-file>
 """;
+
+// TODO(lukechurch): Review UX.
+const String discoverDocumentation = """
+   x-discover Discover Fletch capable devices available
+""";