Updated Readme.md to highlight tableExport link (#3839)

Updated Readme.md to highlight tableExport link, as maybe wasnt as clear that it has its own doco and required steps.

Related issue "can not export data to xlsx" #3805

src/extensions/export/README.md

@@ -2,6 +2,8 @@
 
 Use Plugin: [tableExport.jquery.plugin](https://github.com/hhurz/tableExport.jquery.plugin)
 
+This is an important link to check out as some file types may require extra steps.
+
 ## Usage
 
 ```html