@pawan-patil You are able to modify the source code and use it on your own site. You are only limited if you are intending to redistribute the code. If you intend to, then you must also open-source your changed code.
If in doubt, you'll want to contact a lawyer to review the GPLv3 license that comes with NodeBB.
@agusputra I agree with your suggestion about helper buttons. The open source project Rocket Chat has an unobtrusive way of hinting basic Markdown syntax.
0_1447403857660_markdown_hint1.png
Simply a line of greyed text that becomes regular text on mouseover
0_1447403978825_markdown_hint2.png
It might be useful to have something like this at the bottom of the editor input field.
