[docs] add note about cargo feature flags

[msandy]

Fix for issue #1131

[eladyn]

Thank you! I just realized that maybe the real problem is rather that the documentation is misleading by saying that

To install the resulting binary, run

cargo install --path . --locked

because cargo install doesn't actually install the built binary, but rather compiles it first and then installs it to the correct location. Maybe we could change that to better reflect the actual situation? Would that have solved your problem as well?

[slondr]

Seconding eladyn's point here — the docs are straight up wrong with the "resulting" part, and would probably be better served by making the cargo install section more prominent/higher in the section as to not imply it's part of a different installation process. That should help solve the problem of folks arriving in the repo just trying to install the software as fast as possible.

As an aside, I looked up the current 0.3.4 rollout on repology, and I want to give a special shout-out to Arch, Funtoo, and Void for being the only three distro [families] that are already shipping the new version to users :D