Adjust footer links to repo to be more user-friendly

[arrowtype]

Previously, the footer included

1. A link to “View Contributors,” which itself had replaced credits to only ArrowType & PixelAmbact
2. A basic link to “Source on GitHub”

This updates footer links to:

1. A link to the readme (“How to use Type-X”), in case users are confused, which many may be on first use, especially because this assumes some CSS knowledge. I am still very grateful to contributors here (and I still want to advertise ArrowType), but this seems like a more user-centric use of the real estate.
2. A link to the repo issues (“Submit Feedback”), so users can more directly submit feedback or ask questions. I will still highlight in the Chrome Web Store and in any promotions that this is an open-source extension, but once users have already installed the extension, they have already made their decision, and this is a next action they may wish to take.