docs: document supported spreadsheet formats

README.md

@@ -35,6 +35,13 @@ Options:
 sheetui path/to/file.xlsx # edit/view a spreadsheet
 ```
 
+### Supported files
+
+Currently we only support the [ironcalc](https://docs.ironcalc.com/) xlsx
+features for spreadsheet. CSV import and expor are planned.
+
+### Screenshot
+
 <img src="./assets/screenshot.png" />
 
 ## Reference

docs/index.md

@@ -20,6 +20,13 @@ Options:
   -V, --version                        Print version
 ```
 
+## Supported formats
+
+Currently we only support the [ironcalc](https://docs.ironcalc.com/) xlsx
+features for spreadsheet. I plan to handle csv import and export at some point.
+I also might support other export formats as well but for the moment just csv
+and it's variants such as tsv are in the roadmap.
+
 ## User Interface
 
 The sheetui user interface is loosely inspired by vim. It is a modal interface