Merge pull request airbnb#33 from trungk18/Vietnamese-Translation

Update Vietnamese Translation

Author: lencioni

README.md

@@ -299,3 +299,4 @@ If you must use an ID selector in the first place (and you should really try not
   - ![PT-BR](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Brazil.png) **Portuguese**: [felipevolpatto/css-style-guide](https://github.com/felipevolpatto/css-style-guide)  
   - ![ru](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Russia.png) **Russian**: [Nekorsis/css-style-guide](https://github.com/Nekorsis/css-style-guide)
   - ![es](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Spain.png) **Spanish**: [ismamz/guia-de-estilo-css](https://github.com/ismamz/guia-de-estilo-css)
+  - ![vn](https://raw.githubusercontent.com/gosquared/flags/master/flags/flags/shiny/24/Vietnam.png) **Vietnamese**: [trungk18/css-style-guide](https://github.com/trungk18/css-style-guide)