Documenting IE8 support

[petergphillips]

At the bottom of installing-with-npm.md there is a line:

If your service supports Internet Explorer 8, you will need to generate and include a separate stylesheet as well. This links to https://github.com/ministryofjustice/moj-frontend/blob/master/docs/installation/supporting-internet-explorer-8.md, which doesn't exist.

Would be good to include what information is required for IE8 - i.e. from https://github.com/alphagov/govuk-frontend/blob/master/docs/installation/supporting-internet-explorer-8.md