Required Board Manager addition to README.md

Issue:

Following the directions for HDK/Milli5-Arduino CoAP Server/ I was able to verify the image, however when attempting to upload the board I received the following error

```
java.io.IOException: Cannot run program "{runtime.tools.bossac-1.7.0.path}/bossac": error=2, No such file or directory
```

Solution:

From board manager install the `Arduino SAMD boards`.