Documentation: Add systemd to the list of applications in your .app.src file

[codecov[bot]]

Codecov Report

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

Project coverage is 89.26%. Comparing base (34042c0) to head (3ec80d6). Report is 14 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #37 +/- ## ======================================= Coverage 89.26% 89.26% ======================================= Files 8 8 Lines 354 354 ======================================= Hits 316 316 Misses 38 38 ``` | [Flag](https://app.codecov.io/gh/hauleth/erlang-systemd/pull/37/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%C5%81ukasz+Jan+Niemier) | Coverage Δ | | |---|---|---| | [otp-23](https://app.codecov.io/gh/hauleth/erlang-systemd/pull/37/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%C5%81ukasz+Jan+Niemier) | `89.26% <ø> (ø)` | | | [otp-24](https://app.codecov.io/gh/hauleth/erlang-systemd/pull/37/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%C5%81ukasz+Jan+Niemier) | `89.26% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%C5%81ukasz+Jan+Niemier#carryforward-flags-in-the-pull-request-comment) to find out more.

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