Doc/add spaced app as a reference

[ishiko732]

I would like to add spaced as a reference because it has modern flashcard features, even though it is still in the development stage.

• https://github.com/zsh-eng/spaced/issues/31

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (0fdaa11) to head (0837e58).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #87 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 7 7 Lines 352 352 Branches 44 44 ========================================= Hits 352 352 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.