Add a mention of *caret* mode to the shortcuts (Vimium Help) page

Hi,

First off let me say thank you for an absolutely awesome plugin - I couldn't use Chrome without it. I've been using it for years.

Until recently one thing that still required me to use the mouse was selecting and copying text. Looking on the help page I saw Visual mode mentioned but whenever I used it the selection would start at an arbitrary point and I could find no way to move the cursor around with the movement keys. Given the well thought out design of the rest off the plugin I thought there must be some way to do this and after quite a bit of searching I found some github issue discussions that eventually mentioned caret mode. Armed with caret mode I can now finally do away with the mouse completely, thank you!

I thought that perhaps you can just mention caret mode more prominently in the documentation? Perhaps give it its own section and heading on the Visual Mode page? Also, if it could be mentioned on the shortcuts page (Vimium Help page when pressing ? in normal mode) since "v c" effectively takes you to caret mode.

On the Visual Mode under Tips you could perhaps also mention that using Find Mode is a good way to navigate to the start of your intended selection. This might be obvious to some but I somehow didn't think of it until I read some of the Github issues.

Thanks again!

Start with the Visual Mode wiki page and the Vimium Help shortcuts page referenced in the issue. Review how Visual mode and the “v c” shortcut are currently described, then update the documentation to mention caret mode and the suggested Find Mode navigation tip. Done means both requested pages clearly explain how caret mode supports text selection.