Update README regarding lychee checks on new files

[bparks13]

When running the lychee link checking step locally, it can fail if new files have been created or renamed but have not been pushed to the remote repo yet.

This comes from the newly built HTML page containing a self-referential link in the Edit this page button, but the link it points to is hosted on Github and therefore does not exist if the newly added or renamed file has not been pushed yet. This is normal and expected behavior, and as far as I am aware cannot be filtered out to distinguish between a new local file and a previously existing remote file.

While this is not a solution, we will need to update the README to note this issue so that developers do not spend an undue amount of time trying to troubleshoot this particular issue.

There are no adverse effects on building the pages locally, everything can be tested locally without issue. This will only show up if you try to click on the Edit this page button in a local build, since it will go to a broken Github link. There isn't really a good use case for needing to click on that button from a local build, therefore I believe that noting this in the README will be sufficient.

Below is a snippet of what the error looks like for reference: