Add notice about ghmarkdown that Github's servers are used

`ghmarkdown` uses Github's markdown rendering API 
(https://docs.github.com/en/rest/markdown?apiVersion=2022-11-28) to 
generate the page content.

manifest.json

@@ -450,7 +450,7 @@
       "id": "formatter"
     },
     {
-      "description": "Opens a preview of the current markdown file in a browser window *([screenshot](https://user-images.githubusercontent.com/3920290/82754898-f7394600-9dc7-11ea-8278-2305363ed372.png))*",
+      "description": "Opens a preview of the current markdown file in a browser window *([screenshot](https://user-images.githubusercontent.com/3920290/82754898-f7394600-9dc7-11ea-8278-2305363ed372.png))* Note: the page content is generated by sending the markdown file to Github's markdown rendering [API](https://docs.github.com/en/rest/markdown?apiVersion=2022-11-28)",
       "version": "0.1",
       "path": "plugins/ghmarkdown.lua",
       "id": "ghmarkdown",