Document the --offline flag

Author: jfmengels

lib/flags.js

@@ -383,7 +383,9 @@ const flags = [
   {
     name: 'offline',
     boolean: true,
-    sections: null
+    color: chalk.cyan,
+    sections: ['regular'],
+    description: ['Prevent making network calls.']
   },
   gitHubAuthFlag,
   {

test/snapshots/help/default.txt

@@ -91,6 +91,9 @@ You can customize the review command with the following flags:
     --no-details
         Hide the details from error reports for a more compact view.
 
+    --offline
+        Prevent making network calls.
+
     --ignore-dirs <dir1,dir2,...>
         Ignore the reports of all rules for the specified directories.