README: Specify name when installing for 0.20

Without the name it thinks the plugin is called `-url`:
```
./plugin -install -url https://github.com/downloads/leeadkins/elasticsearch-redis-river/elasticsearch-redis-river-0.0.4.zip
-> Installing -url...
Trying https://github.com/downloads/leeadkins/elasticsearch-redis-river/elasticsearch-redis-river-0.0.4.zip...
Downloading ................................................................DONE
Installed -url
```