How can I create a text box for a note in markdown?

With GitHub, I usually insert a blockquote.

> **_NOTE:_**  The note content.

becomes...

NOTE: The note content.

Of course, there is always plain HTML...

What I usually do for putting alert box (e.g. Note or Warning) in markdown texts (not only when using pandoc but also every where that markdown is supported) is surrounding the content with two horizontal lines:

---
**NOTE**

It works with almost all markdown flavours (the below blank line matters).

---

which would be something like this:

---

NOTE

It works with all markdown flavours (the below blank line matters).

---

The good thing is that you don't need to worry about which markdown flavour is supported or which extension is installed or enabled.

EDIT: As @filups21 has mentioned in the comments, it seems that a horizontal line is represented by *** in RMarkdown. So, the solution mentioned before does not work with all markdown flavours as it was originally claimed.