add sparse repo limitation

Author: extrawurst

README.md

@@ -83,6 +83,7 @@ These are the high level goals before calling out `1.0`:
 
 ## 5. <a name="limitations"></a> Known Limitations <small><sup>[Top ▲](#table-of-contents)</sup></small>
 
+- no sparse repo support (see [#1226](https://github.com/extrawurst/gitui/issues/1226))
 - no support for GPG signing (see [#97](https://github.com/extrawurst/gitui/issues/97))
 - no git-lfs support (see [#1089](https://github.com/extrawurst/gitui/discussions/1089))
 - *credential.helper* for https needs to be **explicitly** configured (see [#800](https://github.com/extrawurst/gitui/issues/800))