Allow presets to create README.md

[tobyzerner]

What problem does this feature solve?

When creating a project, the README.md file is generated after the preset/plugins have been invoked. So even if a preset or a plugin renders a README.md file in its generator, it gets overwritten by the one generated by vue create.

What does the proposed API look like?

The simplest solution would probably be just to check if a README.md file already exists before generating a new one.

[nickbreen]

As a workaround, onCreateComplete fires after README.md is written to disk. E.g.

api.onCreateComplete(() => fs.unlink('README.md))