Add "good first issue" to the issue tracker for low-hanging fruit, make a call for participation

[Shnatsel]

There seems to be plenty of low-hanging fruit on the issue tracker:

https://github.com/image-rs/image/issues/1818
#1825
#1829
#1885

• a lot of low-hanging fruit in the HDR decoder, described at Unnecessary memset()+memcpy() in HDR decoding #1882 (comment)

And that's just from the first page! Plus there's a bunch of issues that are more complicated but could really use attention like all the WebP decoding failures and incorrect results.

I understand the maintainers already have their hands full, so what do you think about a community call for participation? I've seen good contributions come out of good-first-issue tags, and the rest can be tagged help-wanted. I can write up something to post on Reddit with the links. I can tag the issues too (if you give me access to the repo; I'm in the org but don't have access to image bug tracker).