readme suggestion: crosslink uymerge #2

There is a new FOSS tool for Unity file merging, uymerge:

"A structural 3-way merge tool for Unity YAML assets. One dependency-free static binary per platform, registered directly as a git merge driver."

It's written in Rust and may be a good fit for some users. Maybe it can be referenced in the readme?

Related: lgarczyn/uymerge#1