A bit of fixes

I made a bit of fixes.
- [x] Renamed `changelog.md` → `CHANGELOG.md`.
- [x] Formatted README.
- [x] Removed unnecessary instruction steps from README.
- [x] Renamed `Screenshots/` → `img/` and lowercased all screenshots. Reasons:
 - It's easier to type lowercase things.
 - It's more semantic when is screenshots in `screenshot-*` pattern.
 - It's stupid to call directory `Screenshots/` if it contains icon.
- [x] Modified `src/brightness.desktop` to apply with new names.