📝 Updating the description in the browser stores and README

[Lyokolux]

Hello Team 👋

I think the extension changed enough since the first publication. The descriptions on the stores and in the README should be updated, including:

• [x] a description of the required permissions: what are they used for ? (related to #75)
• [x] a part about the config available. At the 2021-04-23, these features are configurable:
  • the Dark mode
  • close the extension on a gitmoji copy
  • show the description of a gitmoji on hover or focus
• [x] how to use the extension with the shortcuts
• [x] README updated
• [ ] Descriptions in the stores updated (#79)
  • [ ] Chrome → html format is not possible, redirect to the README instead
  • [x] Firefox

[Lyokolux]

Task "Descriptions in the stores updated" not done yet. Here the updated page on Firefox: https://addons.mozilla.org/fr/firefox/addon/gitmoji-browser-extension/ @johannchopin please update the description on the chome store.

[johannchopin]

Chrome store doesn't support html description so I will just leave the actual one 👍