Add careers url for Obsidian

Author: myShoggoth

donations/sponsors/obsidian.markdown

@@ -4,4 +4,5 @@ logo: /assets/images/sponsors/obsidian-systems/obsidian-systems-683.png
 srcset: /assets/images/sponsors/obsidian-systems/obsidian-systems-200.png 200w, /assets/images/sponsors/obsidian-systems/obsidian-systems-400.png 400w, /assets/images/sponsors/obsidian-systems/obsidian-systems-683.png 683w
 externalUrl: https://obsidian.systems/
 level: Applicative
+careersUrl: https://www.reddit.com/r/haskell/comments/m2tqca/jobs_obsidian_systems_is_hiring/
 ---