Mention dynamic workspace in various places

* README.md:
* exwm.el: Update README and comments.

* exwm-core.el (exwm-mode-menu exwm-mode-map): Add menu entries for
dynamic workspace feature.

README.md

@@ -1,17 +1,20 @@
 # Emacs X Window Manager
 
-EXWM (Emacs X Window Manager) is a full-featured tiling X window manager for
-Emacs built on top of [XELB](https://github.com/ch11ng/xelb).
+EXWM (Emacs X Window Manager) is a full-featured tiling X window manager
+for Emacs built on top of [XELB](https://github.com/ch11ng/xelb).
 It features:
 + Fully keyboard-driven operations
 + Hybrid layout modes (tiling & stacking)
-+ Workspace support
++ Dynamic workspace support
 + ICCCM/EWMH compliance
 + (Optional) RandR (multi-monitor) support
-+ (Optional) system tray
++ (Optional) Built-in system tray
 
-Please check the [User Guide](https://github.com/ch11ng/exwm/wiki)
-for more details.
+Please check out the
+[screenshots](https://github.com/ch11ng/exwm/wiki/Screenshots)
+to get an overview of what EXWM is capable of,
+and the [user guide](https://github.com/ch11ng/exwm/wiki)
+for a detailed explanation of its usage.
 
-**Note**: If you install EXWM from source, you need to manually install XELB
-(either from source or GNU ELPA).
+**Note**: If you install EXWM from source, it's recommended to install
+XELB also from source (otherwise install both from GNU ELPA).