Update 100-bem.md

Add info and resources of BEM

content/roadmaps/100-frontend/content/108-css-architecture/100-bem.md

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-# Bem
+# Bem
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+The Block, Element, Modifier methodology (commonly referred to as BEM) is a popular naming convention for classes in HTML and CSS. Developed by the team at Yandex, its goal is to help developers better understand the relationship between the HTML and CSS in a given project.
+
+- Article BEM 101: https://css-tricks.com/bem-101/
+- BEM Official Website: https://en.bem.info/
+- BEM Documentation: https://en.bem.info/methodology/quick-start/ 
+- BEM Tutorials: https://en.bem.info/tutorials/