Update README.rst

Clarification concerning yarn. On some systems "yarn - scenario testing of Unix command line tools" is preinstalled.

README.rst

@@ -30,6 +30,9 @@ To set up a new instance perform the following steps:
     or alternatively with the make task::
 
      make install
+    
+    Some systems (like Ubuntu Xenial) come with a preinstalled  "yarn - scenario testing of Unix command line tools". Using this will **not** work. 
+    The [yarn package manager](https://yarnpkg.com/en/docs/install) is needed.
 
 #.  Set up a Postgres 9.5 database. If you have docker installed the easiest way is to just run it in a docker container, like this::