Add a 404 page

[Potherca]

A 404.html page needs to be added to avoid the github default 404 users will encounter on mishaps. This will most often be the case when a githup project does not have a gh-pages branch.

One question that needs addressing is that, when a project on github does have a gh-pages branch, and a user goes 404, will it show the 404 from here or do we need to add our 404.html to all of our projects?

[Potherca]

If I understand this correctly:

Custom 404s will only work if a custom domain is used, otherwise the User Pages 404 is used.

We should be fine untill we start adding custom domains to project gh-pages branches.