Manual.md: add question on GTK_CSD=0

Manual.md

@@ -35,6 +35,8 @@
     - [How do I use Emotes in chats?](#how-do-i-use-emotes-in-chats)
     - [How do I use Emotes from others?](#how-do-i-use-emotes-from-others)
   - [Bug Reporting with logs](#bug-reporting-with-logs)
+  - [FAQ - Frequently Asked Questions](#faq---frequently-asked-questions)
+    - [I use tiling WM, how do I disable the title bar?](#i-use-tiling-wm-how-do-i-disable-the-title-bar)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
@@ -97,3 +99,9 @@ If possible, please also include a screenshot!
 ## Bug Reporting with logs
 _Please also read the bug reporting basics first!_
 
+## FAQ - Frequently Asked Questions
+
+### I use tiling WM, how do I disable the title bar?
+
+It should get disabled when you specify environment variable `GTK_CSD=0`.
+