Document the requirement for a recent setuptools

That is required by html5lib (pulled by xhtml2pdf). Otherwise, the installation
is broken (html5lib triggers an error).

Listing it in
requirements.txt is not enough : it requires to be already up-to-date when "pip
install -r ..." is ran.

README.md

@@ -64,7 +64,11 @@ and `libjpeg-dev` packages.
 
     sudo apt-get install python-dev python-pip libldap2-dev libpq-dev libsasl2-dev libjpeg-dev
 
-Then run:
+You need a setuptools >= 18.5. If you don't meet that requirement (Debian Jessie does not), run:
+
+    pip install setuptools>=18.5
+
+In any case, you then need to install coin python dependencies:
 
     pip install -r requirements.txt