Document "rs" keyword on the actual extension pages, too

[peter-kehl]

Thank you for rust-search-extension.

https://github.com/huhu/rust-search-extension and https://rust.extension.sh do mention "rs" keyword. However, https://addons.mozilla.org/en-CA/firefox/addon/rust-search-extension/ (and probably the extension pages for other browsers, too) doesn't mention "rs" at all. To make it easier for the users, please mention "rs" at the extension pages, too. (I didn't know how to use it, until a friend has showed me.)

This may be similar to #72 but may not be the same (since I don't fully understand #72).

[Folyd]

Hi @peter-kehl. Hugely thanks for your feedback. We indeed need good instruction for the new users in the extensions pages. I'll improve this soon. 🍺