docs: add docs

README.md

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+# express-yup-middleware
+Middleware to validate incoming data in express endpoints using Yup's dead simple schema validation.
+
+[![Build Status](https://travis-ci.org/flasd/express-yup-middleware.svg?branch=master)](https://travis-ci.org/flasd/express-yup-middleware) 
+[![Coverage Status](https://coveralls.io/repos/github/flasd/express-yup-middleware/badge.svg?branch=master)](https://coveralls.io/github/flasd/express-yup-middleware?branch=master) 
+[![npm version](https://badge.fury.io/js/express-yup-middleware.svg)](https://www.npmjs.com/package/express-yup-middleware) 
+[![npm downloads per month](https://img.shields.io/npm/dm/express-yup-middleware.svg)](https://www.npmjs.com/package/express-yup-middleware)
+
+## Instalation
+Install the latest version of express-yup-middleware:
+```
+yarn add express-yup-middleware
+
+// if you are feeling old school
+npm install express-yup-middleware --save
+```
+
+Now you just import it as a module:
+
+```javascript
+const createValidation = require('express-yup-middleware');
+
+// ou, em ES6+
+
+import createValidation from 'express-yup-middleware';
+```
+This module is [UMD](https://github.com/umdjs/umd) compliant, therefore it's compatible with  RequireJs, AMD, CommonJs 1 & 2, etc. It would even work on the browser, but you probably should only use it in NodeJs.
+
+Also, this comes with type definitions in Typescript!
+
+## API & Usage.
+#### createValidation();
+Method signature:
+```typescript
+createValidation(
+  schema: Schema<any> | any,
+  options?: ExpressYupMiddlewareOptions
+): (req: Request, res: Response, next: NextFunction) => Promise<void>;
+```
+```typescript
+import createValidation from 'express-yup-middleware';
+import express from 'express';
+import * as Yup from 'yup';
+
+const validate = createValidation({
+  name: Yup
+    .string('Name must be a string!')
+    .min(3, 'Name too short!')
+    .required('Name is required!')
+});
+
+const app = express();
+
+app.post('/save-name', [validate, (req, res) => {
+  /* req.body.name has been validated! */
+}]);
+```
+
+
+#### Args
+##### schema: Schema<any> | any,
+
+This first argument passed to `createValidation` is a Yup schema or an Object that can will transformed into one.
+All are valid:
+
+```typescript
+/* here req.body must be an object */
+createValidation({
+  name: Yup.string().required(),
+});
+
+// or
+/* here req.body must also be an object */
+createValidation(Yup.object().shape({
+  name: Yup.string().required(),
+}));
+
+// or
+/* here req.body must be an array */
+createValidation(Yup.array());
+```
+
+
+##### options?: ExpressYupMiddlewareOptions
+The second argument passed to `createValidation` is an optional configuration object:
+
+```typescript
+type ExpressYupMiddlewareOptions = {
+  // Yup schema.validate(options) object. Check their documentation! (link below)
+  /* Defaults are:
+   * {
+   *   strict: true,
+   *   stripUnknown: true,
+   *   abortEarly: false,
+   * }
+   */
+  validateOptions?: ValidateOptions;
+
+  // Whenever validation fails, how to send the response to client?
+  responseOptions?: {
+      // res.status(errorCode), defaults is 422 (Unprocessable Entity)
+      errorCode?: number;
+      // should we return errors to the client? default is true
+      returnErrors?: boolean;
+
+      // If you need to transform errors returned from Yup before sending them:
+      transformErrors?: (errors: Array<string>) => any;
+  };
+
+  // where in the req to pull entity from, default 'body'
+  entityFrom: 'query' | 'body' | 'request';
+
+  // where in the given object is the data to be validated (uses lodash.get to access data)
+  // if you want to only validate nested data (eg: req.body.data), pass 'data'
+  // if 'entityFrom' is 'request' and you want to access, for example, req.files, pass 'files';
+  entityPath?: string | number | Array<string | number>;
+
+  // If you need to transform the entity in some way before validating:
+  transformEntity?: (entity: any) => any;
+};
+```
+
+#### Middleware
+
+When you call `createValidation()` the returned value will be a regular express middleware.
+
+```typescript
+const app = express();
+
+const validateHeader = createValidation(
+  Yup.string().required('Authorization'),
+  { entityFrom: 'request', entityPath: ['headers', 'Authorization'] }
+);
+
+app.use(validateHeader);
+
+const validateLogin = createValidation({
+  email: Yup.string().email().required(),
+  password: Yup.string().required(),
+}, { responseOptions: { errorCode: 401 } });
+
+app.post('/login', [validateLogin, (req, res) => { /* do login */ }])
+
+```
+
+### Copyright & License
+
+Copyright (c) 2020 [Marcel de Oliveira Coelho](https://github.com/flasd) under the [MIT License](https://github.com/flasd/express-yup-middleware/blob/master/LICENSE.md). Go Crazy. :rocket: