The options available in the text editor depend on what you are viewing. If you are looking at a conflict report, nothing will be available. If you are looking at anything which can be edited, the leftmost button brings up a dialog from which you can open a file.
If you are editing a readme, the RTF options will also be available. The wordpad icon next to the open file button toggles the RTF features on and off. Keep it off to generate a smaller readme. If you want to use RTF features, you need to click on it. The other buttons toggle bold, italics, underline and the font size. If you want to use any more advanced RTF features, edit the readme elsewhere and then copy/paste it into here afterwards.
The text editor recognizes standard keyboard shortcuts, such as control-x for cut. You can also cut/copy/paste from a context menu by reight clicking.
You can save the text in the editor using the save button.
You can search the text in the editor using the search button.