feat(legacy-create-video): Add README

Author: santakadev

exercises/legacy_create_video/README.md

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+# Legacy create video
+
+Try to add new features to the legacy code
+
+## User story
+
+There is an HTTP API endpoint to create Codely videos.
+When creating videos, we want to ensure that:
+
+- The words "frontend", "Frontend" and "front-end" must be
+  transformed to "Front-end".
+- If the title contains any whitespace at the beginning or
+the end, it must be removed.
+- If the title contains a final dot, it must be removed.
+
+## Exercise
+
+Available languages:
+
+- [PHP Symfony](base/php-symfony/README.md)