Dead link in main README

[emmanuelperu]

Describe the bug
In the main READMe of the project a link with text "How to play with the POC" in the section "How to play with the POC"

To Reproduce
Steps to reproduce the behavior:

1. Go to README
2. Click on 'How to play with the POC'
3. See error in your browser

Expected behavior
The link would refers to page docs/ropsten.md.