minor #13724 Add link to SymfonyCasts React.js tutorial (abdounikarim)

This PR was submitted for the 5.0 branch but it was squashed and merged into the 3.4 branch instead.

Discussion
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Add link to SymfonyCasts React.js tutorial

Hello,

I just add a link to the tutorial on SymfonyCasts.

Commits
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e0a9f9f Add link to SymfonyCasts React.js tutorial

Author: javiereguiluz

frontend/encore/reactjs.rst

@@ -1,6 +1,11 @@
 Enabling React.js
 =================
 
+.. admonition:: Screencast
+    :class: screencast
+
+    Do you prefer video tutorials? Check out the `React.js screencast series`_.
+
 Using React? First enable support for it in ``webpack.config.js``:
 
 .. code-block:: diff
@@ -19,3 +24,5 @@ install any missing dependencies. After running that command and restarting
 Encore, you're done!
 
 Your ``.js`` and ``.jsx`` files will now be transformed through ``babel-preset-react``.
+
+.. _`React.js screencast series`: https://symfonycasts.com/screencast/reactjs