docs: Add a mention that custom linkers are not officially supported yet

[marc2332]

Some people might have unexpected behaviors (like segmentation faults!) when developing freya apps with a custom linker such as mold. There should be a mention of this in the docs.

[Andrew15-5]

Don't put this in you're cargo config.toml, kids! (not yet!)

[target.x86_64-unknown-linux-gnu]
rustflags = [
  "-C", "link-arg=-fuse-ld=mold",
]