Document F-Droid availability

[kisst]

Link Clear is now live on F-Droid as app.linkclear (v1.0.3 / versionCode 10003). The docs still described it as pending, so this brings them in line.

Changes

README.md

• Add an F-Droid version badge (shields.io/f-droid/v/app.linkclear).
• Lead the download call-to-action with the F-Droid listing, keeping the direct APK link alongside it.
• Link the F-Droid listing from the "Install & build" section instead of just naming it.
• Add a short note on what the reproducible build means for users.

docs/INSTALL.md

• Replace the "not yet published there" F-Droid section with real install instructions, and promote it to the recommended method.
• Reframe Obtainium as the option for tracking GitHub Releases directly, rather than "the recommended way to install today" — that framing only held while F-Droid was pending.
• Reorder so F-Droid comes before Obtainium.

On the signing-key claim

The merged fdroiddata recipe carries AllowedAPKSigningKeys: 7e489f6b..., matching docs/fdroid-metadata/app.linkclear.yml in this repo — so the Path 2 (reproducible / cross-signed) submission landed as intended. F-Droid republishes the developer signature rather than signing with its own key, which is why the docs now state that F-Droid, Obtainium, and sideloaded installs share one signing key and can be switched between without an uninstall. Worth a second look, since it is the one new user-facing claim here.

Notes

• Docs only — no code, no build files.
• All relative links verified to resolve; all lines within the 160-char MD013 limit from .mdl_style.rb. mdl is not installed locally and CI has no markdown gate, so the linter itself was not run.