Improve DX when .scalaunfmt.conf is not found

[exoego]

I ran into this issue when I tried to use scalaunfmt.

Steps to reproduce

1. Install coursier.
2. Install scalaunfmt by the below command (from README)

coursier bootstrap com.github.tanishiking:scalaunfmt_2.12:0.0.3 \
  -r sonatype:releases \
  -o scalaunfmt --standalone --main com.github.tanishiking.scalaunfmt.cli.Cli

3. Run scalaunfmt ./scalaunfmt -v 2.0.0-RC8 ./src
4. Then scalaunfmt abort with Exception in thread "main" java.nio.file.NoSuchFileException: .scalaunfmt.conf.

However, it is not clear what .scalaunfmt.conf should look like from README.
I think it should be documented.
Well, it is documented.

It also would be helpful if scalaunfmt

• show more helpful message something like .scalaunfmt.conf is missing. See <URL to example> ,
• use default settings if no .scalaunfmt.conf

from developer experience POV.

[exoego]

Ah, I finally found the example content for .scalaunfmt.conf in https://github.com/tanishiking/scalaunfmt#configuration