Make index.html the default instead of src/index.html

[thedodd]

Shout-out to @MartinKavik for the report on this one as well. This will definitely be an improvement.

Looks like all Seed and Yew templates have index.html in the root or in other directories like static. So I would suggest to write only index.html to README and also try it as the first option if the parameter isn't provided.
~ Martin

Yes, definitely agree on the above. Will do.

• update CLI commands to default to index.html.
• update README to reflect the new default path.