fix the documentation about frontend dependencies

README.md

 
 You also need `redis-server` package if you want to send email in async mode.
 
-### Frontend dependencies ###
+## Frontend ##
 
 [//]: # ( from https://nodejs.org/en/download/package-manager/#debian-and-ubuntu-based-linux-distributions)
 
 
     npm -v
 
-Then install frontend dependancies:
-
-    npm install
-
 ## Database ##
 
 If you want use PostgreSQL as database engine:
 
 *Note: Now everything is documented to be executed from the tracim directory newly created.*
 
+### Frontend dependencies ###
+
+To install frontend dependencies listed in the file package.json, proceed as follow:
+
+    npm install
+
 ### Setting-up python virtualenv ###
 
 _Reminder : Tracim is developed and tested using python3.4._