chore(docs): add no worksheet relation found to troubleshooting; fixes #104

Author: singerla

README.md

@@ -931,6 +931,7 @@ If you encounter problems when opening a `.pptx`-file modified by this library,
 - **Proprietary/Binary contents** (e.g. ThinkCell): Walk through all slides, slideMasters and slideLayouts and seek for hidden Objects. Hit `ALT+F10` to toggle the sidebar.
 - **Chart styles not working**: If you try to change e.g. color or size of a chart data label, and it doesn't work as expected, try to remove all data labels and activate them again. If this does not help, try to give the first data label of a series a slightly different style (this creates a single data point).
 - **Replace Text not working**: Cut out your e.g. {CustomerName} tag from textbox to clipboard, paste it into a plaintext editor to remove all (visible and invisible) formatting. Copy & paste {CustomerName} back to the textbox. (see [#82](https://github.com/singerla/pptx-automizer/issues/82) and [#73](https://github.com/singerla/pptx-automizer/issues/73))
+- **No related chart worksheet**: It might happen to PowerPoint to lose the worksheet relation for a chart. If a chart gets corrupted by this, you will see a normal chart on your slide, but get an error message if you try to open the datasheet. Please replace the corrupted chart by a working one. (see [#104](https://github.com/singerla/pptx-automizer/issues/104)) 
 
 
 If none of these could help, please don't hesitate to [talk about it](https://github.com/singerla/pptx-automizer/issues/new).