Installer Document Update

Hi,

I am referring to this page: [https://certbot.eff.org/instructions](https://certbot.eff.org/instructions).

A new entry for “Apache” and the System selection list “SLES 15.1” could be added.
Below I list the commands for point 2 “Install Certbot” - it’s almost the same like for openSUSE Leap 15.0 but it needs commands which add the modules so `zypper` can install `certbot` and the depending packages.

`SUSEConnect -p PackageHub/15/x86_64`
`SUSEConnect -p sle-module-python2/15.1/x86_64`
`SUSEConnect -p sle-module-public-cloud/15.1/x86_64`
`zypper install python3-certbot python3-certbot-apache`

The other points can remain as they are.