doc: Update for CMake-based build system

Co-authored-by: Lőrinc <pap.lorinc@gmail.com>
Co-authored-by: pablomartin4btc <pablomartin4btc@gmail.com>

contrib/guix/README.md

@@ -261,6 +261,7 @@ details.
 
   - `guix` build commands as in `guix shell --cores="$JOBS"`
   - `make` as in `make --jobs="$JOBS"`
+  - `cmake` as in `cmake --build build -j "$JOBS"`
   - `xargs` as in `xargs -P"$JOBS"`
 
   See [here](#controlling-the-number-of-threads-used-by-guix-build-commands) for