feat: added lesson 1 description

Author: NataliaTepluhina

src/tutorial/src/step-1/description.md

@@ -2,4 +2,33 @@
 
 For now, our component renders some static data. Try to change a number 3 in the code to something different to see how the rendered result changes.
 
-Let's add some dynamic data. Within component `data` return statement, add a property `colorsNumber`.
+How to make it dynamic? We need to create our first _reactive_ property `colorsNumber`
+
+<div class="options-api">
+
+```js
+data() {
+  return {
+    colorsNumber: 3
+  }
+}
+```
+
+</div>
+
+<div class="composition-api">
+
+```js
+import { ref } from 'vue'
+const colorsNumber = ref(3)
+```
+
+</div>
+
+Now, we can use it in our template instead of a static number:
+
+```html
+<h2>Number of colors: {{ colorsNumber }}</h2>
+```
+
+Try to change the `colorsNumber` and check how it immediately updates the rendered HTML.