update README.md.

README.md

 This is a sample HTML/CSS/JavaScript that strictly follows our naming convention.
 
-Reference for the guideline of CSS :
+BEM
+---
+
+For CSS class naming, we use [BEM](https://en.bem.info/) so that we can be sure that which rule will affect which part and also which rule will NOT affect any intended part.
+
+Once tring BEM seriously in a complicated web site, we often face similar questions. Here are resources to answer such questions.
 
 * https://www.smashingmagazine.com/2016/06/battling-bem-extended-edition-common-problems-and-how-to-avoid-them/
 * http://csswizardry.com/2013/01/mindbemding-getting-your-head-round-bem-syntax/
+
+LESS
+----
+
+We use [LESS](http://lesscss.org) for faster development of CSS and also for easier maintenance, like changing colours at once etc.
+
+Here are some guidelines of the usage of LESS :
+
+* You can use the latest released version, i.e. the latest syntax.
+* You don't need to care the size of generated CSS. We can anyway use optimiser to grouping similar rules.
+* If you are not sure which you should choose between 'mixin' and 'extend', always use 'mixin'.