Allow both index.md for VuePress root and README.md for Git info

[benhaynes]

Feature request

What problem does this feature solve?

Allows for a VuePress index page (/index.md -> /index.html) in addition to the common /README.md for the repository's info file. This would allow you to have a custom/informational README file displayed on git services such as GitHub instead of the VuePress homepage with YAML front-matter.

What does the proposed API look like?

I don't know if the API would change based on this feature.

How should this be implemented in your opinion?

The /index.md should take priority over the /README.md in the root when generating the /index.html file (the opposite is true now, with README taking priority). If this is too much of a breaking change, then an option or flag could be added to the VuePress config.js to manually set the default index file to build.

Are you willing to work on this yourself?**

I am willing to offer my time, but wouldn't know how to implement the change itself.

[ulivz]

This feature has been supported by #23