Highlighting Sub topic in a post?

But am looking for a plugin or a code which can help me with adding
this note every time I write a post.

A shortcode to add that “note” is trivial.

function note_sc_wpse_107275($atts,$content) {
  return '<p class="note"><strong>'.$content.'</strong></p>';
}
add_shortcode('note','note_sc_wpse_107275');

Then just add [note]Note content[/note] to your post body.

That will give you exactly what you see on that site, assuming that I am looking at the right piece of it. By default, it won’t look like anything special. You will need to add style rules to decorate it however you like.