update 1.0 goals

Author: Stephan Dilly

README.md

@@ -79,6 +79,7 @@ These are the high level goals before calling out `1.0`:
 * notify-based change detection ([#1](https://github.com/extrawurst/gitui/issues/1))
 * interactive rebase ([#32](https://github.com/extrawurst/gitui/issues/32))
 * popup history and back button ([#846](https://github.com/extrawurst/gitui/issues/846))
+* submodule support ([#1087](https://github.com/extrawurst/gitui/issues/1087))
 
 ## 5. <a name="limitations"></a> Known Limitations <small><sup>[Top ▲](#table-of-contents)</sup></small>