Add Firefox add-on link to README and InstallAkita

[sharon-wang]

Fixes: https://github.com/esse-dev/akita/issues/133

Adds the Firefox add on installation link https://addons.mozilla.org/en-US/firefox/addon/akita/ to our README and install instructions, and also adds a badge to our README for Firefox.

Previews

• README: https://github.com/sharon-wang/akita/blob/addfirefoxbadge/README.md
• InstallAkita: https://github.com/sharon-wang/akita/blob/addfirefoxbadge/docs/InstallAkita.md

Badge Preview