Add deprecation warning to the book

Signed-off-by: Robert Vojta <rvojta@me.com>

docs/mdbook/src/SUMMARY.md

@@ -1,4 +1,8 @@
 # Crossterm
+
+> **WARNING**: This book is deprecated, no longer maintained and will be
+> removed soon.
+
 This book explains how crossterm works, and how you can use different functionalities. 
 We look at how to turn feature flags features on and off, how to style the terminal with colors and attributes, 
 how to read user input and how to make using crossterm easier.

docs/mdbook/src/command.md

@@ -1,4 +1,8 @@
 # Command API
+
+> **WARNING**: This book is deprecated, no longer maintained and will be
+> removed soon.
+
 The command API makes the use of crossterm much easier and offers more control over when and how a command such as moving the cursor is executed.
 
 The command API offers:

docs/mdbook/src/feature_flags.md

@@ -1,3 +1,6 @@
+> **WARNING**: This book is deprecated, no longer maintained and will be
+> removed soon.
+
 From `crossterm 0.6` you are able to use feature flags. 
 
 With feature flags, you can pick the features you want which reduces the size of the library and could prevent you from having unnecessary dependencies.

docs/mdbook/src/input.md

@@ -1,3 +1,6 @@
+> **WARNING**: This book is deprecated, no longer maintained and will be
+> removed soon.
+
 Crossterm provides a way to work with the terminal input. We will not cover the basic usage but instead asynchronous and synchronous reading of input.
 Please check out these [examples](https://github.com/crossterm-rs/crossterm/blob/master/examples/input.rs) for reading a line or a character from the user.
 

docs/mdbook/src/screen.md

@@ -1,3 +1,6 @@
+> **WARNING**: This book is deprecated, no longer maintained and will be
+> removed soon.
+
 ## Screen Buffer
 A screen buffer is a two-dimensional array of characters and color data to be output in a console window. 
 A terminal can have multiple of those screen buffers, and the active screen buffer is the one that is displayed on the screen.

docs/mdbook/src/styling.md

@@ -1,5 +1,8 @@
 # Styling Module
 
+> **WARNING**: This book is deprecated, no longer maintained and will be
+> removed soon.
+
 Crossterm provides options for you to style your text and terminal. Take for example coloring output and applying attributes.
 
 **Color support**