Add axens scheduler to

docs/learn/concepts/addons.md

@@ -2,15 +2,14 @@
 A collection of third-party jecs addons made by the community. If you would like to share what you're working on, [submit a pull request](https://github.com/Ukendio/jecs)!
 
 # Debuggers
-
 ## [jabby](https://github.com/alicesaidhi/jabby)
-
 A jecs debugger with a string-based query language and entity editing capabilities.
 
 # Schedulers
+## [lockstep scheduler](https://gist.github.com/1Axen/6d4f78b3454cf455e93794505588354b)
+A simple fixed step system scheduler.
 
 ## [sapphire-jecs](https://github.com/Mark-Marks/sapphire/tree/main/crates/sapphire-jecs)
-
 A batteries-included [sapphire](https://github.com/mark-marks/sapphire) scheduler for jecs
 
 This page takes wording and terminology directly from Bevy's [assets page](https://bevyengine.org/assets/)