[guide] Add a basic ModifyGuide

visidata/guides/ModifyGuide.md

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+# Adding, Editing, Deleting Rows
+
+In addition to cutting and pasting (see **Clipboard Guide**), rows can be created from scratch, edited and deleted.
+
+- {help.commands.add-row}
+- {help.commands.add-rows}
+
+- {help.commands.delete-row}
+- {help.commands.delete-selected}
+
+- {help.commands.sysedit-selected}
+- {help.commands.edit_cell}
+
+While editing a cell, press `Ctrl+G` to cycle through the sidebar guides to see the **Input Keystroke Help**.
+
+A sample of available commands:
+    - [:code]Ctrl+O[/] to edit the cell in a text editor.
+    - [:code]Tab[/] to finish editing and go to the next cell
+    - [:code]Shift+Tab[/] to finish editing and go to the previous cell
+
+Modifying a sheet displays a `[M]` in the lower right status.
+
+- {help.options.quitguard}
+
+## Deferred Sheets
+
+Some sheet types support *deferred* saving. Examples include **SQLiteSheet** and **Dir Sheet**. Modifications are colored: green for added rows, red for deleted rows, and yellow for modified cells.
+
+Commit the sheet to reflect changes on the filesystem.
+
+- {help.commands.commit_sheet}