Created Keyboard API (markdown)

Keyboard-API.md

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+# Keyboard API
+
+[![Doc Status](https://godoc.org/github.com/mum4k/termdash/keyboard?status.png)](https://godoc.org/github.com/mum4k/termdash/keyboard)
+
+The [keyboard](https://github.com/mum4k/termdash/tree/master/keyboard) package exposes types that represent keys on the keyboard.
+
+This is a data-only package which provides types that are accepted by the infrastructure and widgets in multiple locations.
+
+The public API surface of this package consists of the following:
+
+## [keyboard.Key](https://github.com/mum4k/termdash/blob/b8591308b93dbd58d97e3a83998e180ecc17cf11/keyboard/keyboard.go#L18-L22)
+
+The **keyboard.Key** type represents keys on the keyboard. The package provides predefined values for some of the keys which can be used directly, e.g:
+
+```go
+var key Keyboard
+key = keyboard.KeyEnter
+```
+
+For all other keys like regular letters, the character that represents the letter can be directly assigned to the Key type.
+
+```go
+var key Keyboard
+key = 'a'
+```