Minor readme formatting

I've made the readme prettier by using the github extensions for markdown. Code now looks like; 

```go
package main

import (
	_ "github.com/lib/pq"
	"database/sql"
)

func main() {
	db, err := sql.Open("postgres", "user=pqgotest dbname=pqgotest sslmode=verify-full")
	// ...
}
```

rather than:

    package main

    import (
      _ "github.com/lib/pq"
      "database/sql"
    )

    func main() {
      db, err := sql.Open("postgres", "user=pqgotest dbname=pqgotest sslmode=verify-full")
      // ...
    }