README.md: run tests command doesn't work.

```bash
~/src/github.com/wolfgangmeyers/go-warc$ go test warc warc/utils
```
This isn't working this way.

You shall either run
```bash
~/src/github.com/wolfgangmeyers/go-warc$ go test ./warc ./warc/utils
```
or run
```bash
~/src/github.com/wolfgangmeyers/go-warc$ go test ./...
```

Also, make sure you have **`gopkg.in/check.v1`** installed.

**go version go1.10.1 linux/amd64**
