Update usage instructions

Problem/Motivation

As per the main module page:

Usage
Make sure you have Views module v7.x-3.4 or higher installed.

This seems to be redundant, as the visible module releases are 8+

Steps to reproduce

Go to https://www.drupal.org/project/views_ical →

Proposed resolution

Have D8+ instructions, if the D7 instructions are to remain then split it out.

Remaining tasks

Have D8+ instructions, if the D7 instructions are to remain then split it out.