Add support for nxxm.

[nxxm](https://nxxm.github.io)

include/ftxui/README.md

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+# 
+* Level 0: terminal output.
+* Level 1: ftxui::Screen               
+* Level 2: ftxui::Node
+* Level 3: ftxui::Component
+
+## Level 0: terminal output.
+  The terminal you know, you can append text on it. It is represented by
+  std::cout.
+
+## Level 1: ftxui::Screen               
+  A rectangular grid of characters.
+  Use Terminal::ToString() to append its content into the console.
+
+## Level 2: ftxui::Node
+  A hierarchical set of element.
+  They handle layout and Render themself on the screen.
+  See ftxui/dom/elements.hpp
+
+  You can make implement your own.
+
+## Level 3: ftxui::Component
+  A hierarchical set of component. A component render itself by producing
+  ftxui::Node in Component::Render().
+
+  Some component can handle events:
+    * keyboard
+    * mouse
+    * terminal event
+
+  Implement your own!