Adding archetypes default file

[nonumeros]

If the variable 'theme' is specified on the site configuration file - config.toml in this case - and then e.g. create a new post, an error is returned that the archetypes directory is not found.
Therefore a default.md archetype file must be in place
I think this fixes it - according to the latest modifications from gohugoio/hugo@863a812

[nonumeros]

Let me recap what I said earlier. Note that I didn't try with any other config files other than with the default toml suffix type after a site is created with hugo

Default config.toml for the site would be

 baseURL = "http://example.org/"
 languageCode = "en-us"
 title = "My New Hugo Site"

 theme = "lanyon-hugo"

https://gohugo.io/getting-started/quick-start/#site-configuration

 hugo new <site>
 cd <site>
 hugo new post/post1.md

If theme="lanyon-hugo" is specified on that default config.toml file created by hugo,
then the error Unable to find archetypes directory for theme "lanyon-hugo" is returned.

Hence an archetypes file is needed.