v0.1.1: run tests in CI, add REUSE compliance and CONTRIBUTING

- Add .github/workflows/test.yml, using flatpak-builder's run-tests
  option so PRs and pushes exercise ctest inside the same KDE SDK
  sandbox as release builds.
- Add REUSE.toml covering files that can't carry an inline SPDX header
  (.desktop, keys/*.asc).
- Add CONTRIBUTING.md documenting build/test/release steps.

REUSE.toml

@@ -0,0 +1,12 @@
+version = 1
+SPDX-PackageName = "Kareer"
+SPDX-PackageSupplier = "ToServeTheKing <[email protected]>"
+SPDX-PackageDownloadLocation = "https://github.com/toservetheking/Kareer"
+
+[[annotations]]
+path = [
+    "io.github.toservetheking.Kareer.desktop",
+    "keys/*.asc",
+]
+SPDX-License-Identifier = "GPL-3.0-or-later"
+SPDX-FileCopyrightText = "ToServeTheKing <[email protected]>"