A bit of fixes

I made a bit of fixes.
* Renamed `changelog.md` → `CHANGELOG.md`.
* Formatted README.
* Removed unnecessary instruction steps from README.
* Renamed `Screenshots/` → `img/` and lowercased names of 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.
* Modified `src/brightness.desktop` to apply with new names.