Add pkg-config file. (#642)

This file describes the include and lib location and makes
it possible to use the library in build systems other than
CMake, e.g. in basic Makefiles.

Signed-off-by: Henner Zeller <h.zeller@acm.org>

CMakeLists.txt

@@ -3,6 +3,7 @@ cmake_minimum_required(VERSION 3.11)
 project(ftxui
   LANGUAGES CXX
   VERSION 4.0.0
+  DESCRIPTION "C++ Functional Terminal User Interface."
 )
 
 option(FTXUI_BUILD_DOCS "Set to ON to build docs" ON)