README: also mention CentoOS and RHEL in instructions

The latest supported version of Fedora is now 24, and python3 is
supported on all current versions, so drop references to old Fedora
from the README.

Fixes systemd#47.

README.md

@@ -31,12 +31,12 @@ On openSUSE and SLE
 
 To build from source
 
-On Fedora 21+ with Python 2:
+On CentOS, RHEL, and Fedora with Python 2:
 
     dnf install git python-pip gcc python-devel systemd-devel
     pip install git+https://github.com/systemd/python-systemd.git#egg=systemd
 
-On Fedora 21+ with Python 3:
+On Fedora with Python 3:
 
     dnf install git python3-pip gcc python3-devel systemd-devel
     pip3 install git+https://github.com/systemd/python-systemd.git#egg=systemd