Add info for matrix image messages

[poVoq]

With #36 now basically fixed, there was an somewhat related improvement mentioned that would be still nice to have, see:

42wim#2228

@c-holtermann would you be up for making a PR for that here since you are still active and interested in Matterbridge?

[selfhoster1312]

I added a slightly-modified version of 42wim#2228 in #63 (mixed testing branch) and #61 (matrix helpers)

[poVoq]

Done. Thanks to all involved.