Add dni module

Author: albertfdp

.gitignore

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+node_modules
+coverage

README.md

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 # dni-js
-Compute and validate a Spanish DNI/NIE number
+Compute and validate a Spanish DNI/NIE numbers.
+
+## Install
+
+```
+$ npm install dni-js
+```
+
+## Usage
+
+Validation is based on this [document](http://www.interior.gob.es/web/servicios-al-ciudadano/dni/calculo-del-digito-de-control-del-nif-nie).
+
+```js
+const dni = require('dni-js');
+
+dni.isValid('12345678-Z'); // => true
+```
+
+### API
+
+#### `.dni (number|string)`
+
+Returns a DNI number with the control digit appended in the format `12345678-Z`. If the input is invalid,
+returns `null`.
+
+#### `.nie (number|string)`
+
+Returns a NIE number with the control digit appended in the format `X1234567-L`. If the input is invalid,
+returns `null`.
+
+#### `.getControlDigit (number|string)`
+
+Returns the control digit for the input.
+
+#### `.getLetter (number|string)`
+
+Alias for `getControlDigit`.
+
+#### `.isValid (string)`
+
+Returns `true` or `false` by validating the input.
+
+## License
+
+MIT