Add commented out block to simplify setting default temperature display unit

[matt-sorenson]

Had this thought when I saw #131

I feel like when I was first learning esphome this type of config was hard as I wasn't familiar with using extend. I think some common example configs that the user may want to override could be helpful for on-boarding new users and for existing users it still simplifies it if they want to change a default that a decent number of users will want changed.

Even if #131 is merged in the opposite version of this PR with it set to RESTORE_DEFAULT_ON would be good.

[MallocArray]

You don't necessarily need to use extend to change the temperature unit, as you can do it directly in Home Assistant as well as by pressing the button on the back of the device as mentioned in the main README.md file.

So I think this is extra noise in the main configuration file to add extra commented out information, when there are other methods to accomplish the same goal that are built in.

I wouldn't be opposed to a separate page or maybe a part of the configuration.md that could list some specific examples of things like extend blocks as users find them helpful.