docs: describe a way to install all dependencies at once (#380)

Author: iliyaZelenko

.README/README.md

@@ -20,6 +20,9 @@
 npm install eslint --save-dev
 npm install babel-eslint --save-dev
 npm install eslint-plugin-flowtype --save-dev
+
+# Or all at once: 
+npm install eslint babel-eslint eslint-plugin-flowtype --save-dev
 ```
 
 ## Configuration