Pelican: Add Repology badge to README

[luzpaz]

Summary

A way for users to see what downstream repositories carry pelican

Proposed design

The compact badge looks like so:

The full badge looks like so (but may be too "bulky"):

---

Note: There is also a possibility to add the full badge in to its own section, like so:

### Packaging Status

<details>
  <summary>Expand to see the packaging status of pelican...</summary>

[![Packaging status](https://repology.org/badge/vertical-allrepos/pelican.svg?header=pelican)](https://repology.org/project/pelican/versions)  

</details>

Which gives us:

Packaging Status

Expand to see the packaging status of pelican...

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your participation and understanding.

[ivanhercaz]

@getpelican, any comment about it? It might be a good idea. I will use the compact badge to the right of the PyPI badge.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your participation and understanding.

[luzpaz]

Bump?

[avaris]

I'm OK with the compact one.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your participation and understanding.

[luzpaz]

Bump

[justinmayer]

Thanks to @luzpaz for the contribution and to @ivanhercaz and @avaris for reviewing.