I started to look into the templates for the Hello Friend theme.
If adding stuff to the <head> I’m wondering if there is a typo in the layouts/partials/extended_head.html file.
Should this read
If you want to include any custom html just before </head>, put it in /layouts/partials/microblog_head.html?
The theme was created before the inclusion of microblog_head.html. The microblog_head.html includes things specific to micro.blog such as custom css and plugs.
I would suspect that first line still applies if you want to add something. Or, you can add things via the Meta plugin
These comments were in the file
layouts/partials/extended_head.html
The second line states
Do not put anything in this file - it’s only here so that hugo won’t throw an error if /layouts/partials/extended_head.html doesn’t exist.
“this file” is layouts/partials/extended_head.html 
The plugin would probably do the trick, thanks, but It’s just a little confusing.
It can be very confusing to understand. The extend_head.html is called from the head.html and you will get and error if a file is called and isn’t there.
We can only speculate, but the original author probably wrote that comment as a note to themselves. “Hey, future self, in case you forget what this file is about, it’s just a placeholder that’s intended for theme users to override.”
The idea is that instead of editing that file, the user creates a custom theme and includes their own file, overriding the original one. But technically, nothing is stopping you from editing the original.
Depending on how a developer designs and structures their theme, there might be more than one way to add stuff to the head. In this specific case, the templates extended_head.html, head.html, baseof.html, and even microblog_head.html are potential locations for adding additional head code. The original theme developer intended for you to put stuff in extended_head.html but in the end, it’s up to you.
As @mandaris already pointed out, the Micro.blog way to just add something to the head is via the plug-in.
I understand Hugo and templates a tiny bit. My only confusion is what I think is a typo or a cut and paste error for the path to the file.
In the first line of the comment they say to add your extra stuff to
layouts/partials/extended_head.html
The second line of the comment starts out
Do not put anything in this file
which is … extended_head.html as you can see from the screenshot.
I thought the comment instructions might have been from the micro.blog version of the theme and if there was a preferred way to add the stuff.
@sod clarified that it was probably from the original theme author and the preferred way is by plugin, which is cool. I just like to dig around. I was hand coding HTML and CSS 20 years ago … for fun. 
