Document that WT_HOLOCHAIN_PATH requires a holochain built with unstable-countersigning #678

The README documents WT_HOLOCHAIN_PATH for pointing Wind Tunnel at your own holochain binary,
which is a genuinely useful escape hatch if you aren't set up with Nix. What isn't documented is
that the binary has to be built with particular features, and the failure when it isn't is hard to
read.

Using the released holochain-0.7.0 binaries, a scenario run fails at module build with:

ModuleBuild("agent_activity: Error while importing
  \"env\".\"__hc__accept_countersigning_preflight_request_1\": unknown import")

The cause makes sense once you find it: the workspace Cargo.toml enables hdk with
unstable-functions and unstable-countersigning, so every scenario's zome WASM declares the
countersigning host function, and the conductor has to provide it. flake.nix:62 builds Holochain
with exactly those features (cargoExtraArgs = "--features unstable-functions,unstable-countersigning"),
so anyone using the Nix workflow never sees this. Because the hdk features are set at the
workspace level, this isn't specific to one scenario.

⚠️ The part that cost me the most time: the holochain-unstable-* release asset fails the same
way. Its name makes it look like the one to reach for, but it doesn't carry the countersigning
host function either. That's visible in the binaries without running anything:

gh release download holochain-0.7.0 --repo holochain/holochain \
  -p 'holochain-x86_64-unknown-linux-gnu' -p 'holochain-unstable-x86_64-unknown-linux-gnu'
for b in holochain-x86_64-unknown-linux-gnu holochain-unstable-x86_64-unknown-linux-gnu; do
  echo "== $b"
  strings -n 8 "$b" | grep -oE '__hc__(sleep|accept_countersigning_preflight_request)_1' | sort -u
done

== holochain-x86_64-unknown-linux-gnu
== holochain-unstable-x86_64-unknown-linux-gnu
__hc__sleep_1

__hc__sleep_1 is registered under #[cfg(feature = "unstable-functions")] and
__hc__accept_countersigning_preflight_request_1 under #[cfg(feature = "unstable-countersigning")]
(crates/holochain/src/core/ribosome/real_ribosome.rs:509-516 at tag holochain-0.7.0). So the
"unstable" asset carries unstable-functions but not unstable-countersigning — which is exactly
the one Wind Tunnel's zomes need. Building from source with both features works; it took 13 minutes
on an 8-core box.

A sentence next to the WT_HOLOCHAIN_PATH documentation would have saved me a couple of hours —
something like "the binary must be built with --features unstable-functions,unstable-countersigning;
the released binaries, including holochain-unstable-*, are not.
Happy to open a PR if it'd help, though I can't run the Nomad CI workflow your PR template asks for.

(If the intent is that holochain-unstable-* should carry all unstable features, that'd be one for
the holochain repo rather than here — I've only checked the 0.7.0 assets.)

Environment: wind-tunnel @ e4861457, Holochain 0.7.0, Linux x86_64, not using Nix.