How to add comments into a mustache template?

Mustache documentation suggests to use following for comments:

Comments begin with a bang and are ignored. The following template:
    <h1>Today{{! ignore me }}.</h1>

Will render as follows:    
    <h1>Today.</h1>

Comments may contain newlines.

I assume that in your case you had to use

{{! blah }}

Instead of

{{!--  blah --}}

You are trying to comment on mustache template that also has html in it, so you actually have to add mustache comment where the mustache code is and html comment where the html code is.

.