Add markdown syntax explaination

[djyde]

Cusdis comment is support a subset of Markdown syntax. Should write a document page about it, explain which syntax is support, which is not. And add this document link to the widget, below the reply textarea.

https://github.com/djyde/cusdis/blob/master/service/comment.service.ts#L8-L12

[danisztls]

Does it support all basic markdown-it features?

Can we have emoji support?

[danisztls]

That plugin uses the full set or a subset of GH emoji.

Syntax for future reference:
https://github.com/ikatyang/emoji-cheat-sheet/blob/master/README.md

[djyde]

@lbcnz As the code here https://github.com/djyde/cusdis/blob/master/service/comment.service.ts#L13 I only disable image and link syntax for security.

I will look at the plugin. Thank you!