Update README.md

[cjrace]

This is to undo what I had previously done in #9, and explained in dfe-analytical-services/shiny-template#70.

Having re-read the guidance on renv::install() (and other guidance more generally) to try to troubleshoot issues in deployment pipelines with the alternative install of dfeshiny, it seems that the original method was the recommended one.

Given this, and starting to bang my head against a wall a bit, I switched to trying to work out my original error I was having locally (along with some others). Time to hold my hands up and admit I should have persevered with this more in the first place...

• I had a rogue .Renviron file that was in my onedrive documents folder from years ago setting an outdated GitHub PAT (amongst other things)
• I hadn't considered this as I'd moved to a new laptop recently, and didn't know I had anything set in onedrive folders that would have carried over to the new laptop

It now works with the original instructions. I've added a quick note at the bottom of the README too, to help others who may hit this issue.