Add "png" to the mix

[k-dominik]

Episode 02 uses bmp and jpg to compare lossy and no compression. Would be nice to add PNG to show that for certain images (e.g. just one color as used in the example), the lossless compression works very well.

[tobyhodges]

Since #206 was merged, PNG is mentioned in the table at the end of the episode. However, I think the episode content needs a paragraph or two talking more about PNG and the compression it offers. At the moment, we have a whole subsection about BMP but no mention of PNG outside that table.

[CaptainSifff]

Hi, A long time ago, I wrote a decoder for PNG...
Hence I have the feeling I could say something about it...
Is this still relevant to be added to the lesson?

[quist00]

Yes, I think we would welcome a pull request on this topic if you have the bandwidth for that. If you just wanted to provide the prose for us to work in, I am sure that would still be welcome as well.

[tobyhodges]

This was closed in #217. Thank you @CaptainSifff 🙌