Link to alternative and/or enhanced forks

[willnorris]

imageproxy isn't for everyone. There are other projects with different goals, or forks of imageproxy that add features I won't merge upstream for whatever reason. I've long thought about adding a caddy-style plugin API, but until that happens (if ever), figure out a good, low-maintenance way to link to these other projects and forks. Maybe a wiki page? Maybe GitHub Discussions?

Related: #114