Update index.html

[probinso]

Took a while to track this information down while using your fantastic tool. I think that adding this to the introduction documentation could save people some time.

I assert that this patch is my own work, and to simplify the licensing of the BFG Repo-Cleaner:

(choose 1 of these 2 options)

• I assign the copyright on this contribution to Roberto Tyley
• I disclaim copyright and thus place this contribution in the public domain