mastodon: add related_software_url (Clients) link (#374)

- point to community-maintained https://github.com/tleb/awesome-mastodon
- update link to maintained awesome-mastodon fork, ref. tleb/awesome-mastodon#71
- related #373

software/mastodon.yml

@@ -8,6 +8,7 @@ platforms:
 tags:
   - Communication - Social Networks and Forums
 source_code_url: https://github.com/mastodon/mastodon
+related_software_url: https://github.com/hyperupcall/awesome-mastodon
 stargazers_count: 44569
 updated_at: '2023-11-15'
 archived: false