README: drop distro instructions for python2, drop epel/rhel

Fedora has no python2 package, so drop that.
Similarly, with latest Debian and Opensuse, 'apt-get install
python3-systemd' works, but 'apt-get install python-systemd' doesn't.

(Tested with
podman run -it --rm debian apt-get install python3-systemd
mock -r opensuse-tumbleweed-x86_64 --enable-network -i python3-systemd)

It also seems we never built the package in EPEL, so let's drop
RHEL/CentOS from the list.

If people need to, they can install from sources where the distro
packages are unavailable (and they shouldn't be using python2 anyway
at this point.)

Also add 'apt-get update' into the instructions because w/o that
install does not work in a pristine container.

Fixes #78.

Author: keszybz

README.md

@@ -17,17 +17,22 @@ Installation
 
 This module should be packaged for almost all Linux distributions. Use
 
-On Fedora/RHEL/CentOS
+On Fedora:
 
-    dnf install python-systemd python3-systemd
+    dnf install python3-systemd
 
-On Debian/Ubuntu/Mint
+On Debian/Ubuntu/Mint:
 
-    apt-get install python-systemd python3-systemd
+    apt-get update
+    apt-get install python3-systemd
 
-On openSUSE and SLE
+On openSUSE and SLE:
 
-    zypper in python-systemd
+    zypper in python3-systemd
+
+On Arch:
+
+    pacman -Sy python-systemd
 
 To build from source
 --------------------