Adds 'Intro to Vue 3' course (#3830)

* Adds 'Intro to Vue 3' course

Adds the free 'Intro to Vue 3' course on Vue.js v3 from https://vuemastery.com

* Minor grammatical fix

Changes `Vue.Js` to `Vue.js` on the courses list

free-courses-en.md

@@ -27,6 +27,7 @@
   * [React](#react)
   * [Redux](#redux)
   * [Sails.js](#sailsjs)
+  * [Vue.js](#vuejs)
 * [Linux](#linux)
 * [Lua](#lua)
 * [Machine Learning](#machine-learning)
@@ -281,6 +282,11 @@
 * [Develop web apps in Node.js and Sails.js](https://courses.platzi.com/courses/develop-apps-sails-js/)
 
 
+### Vue.js
+
+* [Intro to Vue 3](https://www.vuemastery.com/courses/intro-to-vue-3/intro-to-vue3)
+
+
 ### Linux
 
 * [Introduction To Linux](https://www.edx.org/course/introduction-to-linux/) - The Linux Foundation (edx)