Bring the README, the docs index and CONTRIBUTING up to 0.17.0

The README said export went "to a folder here or back into the
library", which albums replaced and forbid, and called presets "named
presets" beside a shipped collection of film looks and Lightroom
imports. It now says both, and that a photograph only on the server
opens on its thumbnail with its download's progress. The requirement
count comes from traceability.md's summary: 192, 84% claimed; the
version and release count are left to the release commit.

docs/README.md's manual row gains presets and albums. CONTRIBUTING
said 177 requirements and 826 crates, and the Flatpak manifest 826
crates; the lockfile now holds 849 packages, 822 from the registry.

packaging/flatpak/paris.tourolle.darkroom.yml

@@ -117,7 +117,7 @@ modules:
         # from the state flatpak-builder is managing and not from whatever the
         # host's cargo cache happens to hold.
         CARGO_HOME: /run/build/darkroom/cargo
-      # Cargo fetches 826 crates, and Flathub's builders forbid this — a
+      # Cargo fetches over 800 crates, and Flathub's builders forbid this — a
       # submission there needs `cargo-sources.json` generated by
       # flatpak-builder-tools' `flatpak-cargo-generator.py` from Cargo.lock,
       # listing every crate as its own source, plus a vendored-registry