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Add link to the challenge's page.

README.md

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-My solution for the Frontend Mentor's “Chat app CSS illustration” challenge.<br>
-Mi solución para el desafío “Chat app CSS illustration” de Frontend Mentor.
+<i>English:</i><br>
+My solution for the <a href="https://www.frontendmentor.io/challenges/chat-app-css-illustration-O5auMkFqY">Frontend Mentor's “Chat app CSS illustration” challenge</a>.
+<hr>
+<i>Español:</i><br>
+Mi solución para el <a href="https://www.frontendmentor.io/challenges/chat-app-css-illustration-O5auMkFqY">desafío “Chat app CSS illustration” de Frontend Mentor</a>.