Mention seaborn vs matplotlib

Matplotlib does not provide good enough i.e **heatmaps**, which could hinder plot interpretation. I thought it would be interesting to mention this in order to introduce interested students to better plotting.

I also found a small problem while running `jekyll -t build -d _site` in case the machine does not have make (subcommands should go first):

```
$ jekyll -t build -d _site
Invalid command. Use --help for more information
```

I would argue on removing this comment altogether though... Having machines without `make` should be quite rare, imho, right?
Nice catch.