readme: mention python 3.5+ as a requirement

backend/README.md

@@ -3,6 +3,7 b''
 
 To run the app you need to have meet prerequsites:
 
+- python 3.5+
 - running elasticsearch (2.3+ tested)
 - running postgresql (9.5+ required)
 - running redis

readme.rst

@@ -18,6 +18,7 b' Install the app by performing'
 
 To run the app and configure datastore you need to run:
 
+* python 3.5+
 * elasticsearch (2.2+ tested)
 * postgresql 9.5+
 * redis 2.8+