README: update link to metro extracts

As noted on http://metro.teczno.com/ :
```
This collection is no longer maintained. 

The fine folks at Mapzen have built a newer and more up-to-date version of Metro Extracts. Get data from mapzen.com/metro-extracts and suggest changes at mapzen/metroextractor-cities.
```
Merge pull request #89 from wilsaj/patch-1

README: update link to metro extracts