Add except sentence for docs

docs/design-choices.md

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 # Design choices
 
+Design an ideas behind this library.
+
 ## Polling
 
 This library's goal is to enable a [polling-based](https://en.wikipedia.org/wiki/Polling_(computer_science)) paradigm on input handling. [Inputs](https://en.wikipedia.org/wiki/Input_device) come from mouse, keyboard, gamepad, touch screen, etc. Polling means that you can check for an input's current state anywhere that feels natural in your game code instead of managing them through [events and callbacks](https://en.wikipedia.org/wiki/Event_(computing)).

docs/getting-started.md

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 # Getting started
 
+This guide will show you how to get started with this library.
+
 ## Install
 
 Install using the following dotnet command: