Add an example for reading repos from a file

go/stars/README.md

# Fetch all repos (public, private, org member)
$ token=<personal access token>
$ curl -u heyLu:$token https://api.github.com/user/repos | ./stars
# Fetch repos from a file
$ ./stars < repos.json
...
```
However, note that GitHub's API uses pagination, so ensure you set the