Create index.html

[mulark]

Instead of 404 lets redirect to the calculator (calc.html)

[KirkMcDonald]

The only reason I've hesitated on this is the vague thought that I might use my github pages for something else at some point in the future, and I might want to use the index as, you know, an index. But I can always add this now and change it later if I feel like it. Approved.