Add a basic README

README.md

+# apertis-flatpak-runtime
+
+Reference Flatpak runtime for Apertis.
+
+## Building Locally
+
+This currently needs [a separate branch of
+apertis-flatdeb](https://gitlab.apertis.org/pkg/apertis-flatdeb/-/tree/wip/refi64/T8169-changes),
+and `FLATDEB_REPO` should be set to the path to the local clone of the branch.
+
+The headless and HMI runtimes can be built via:
+
+```
+./flatdeb.sh v2022pre amd64 runtimes runtimes/org.apertis.headless.yaml
+./flatdeb.sh v2022pre amd64 runtimes runtimes/org.apertis.hmi.yaml
+```
+
+This will build both the .Platform and .Sdk variants of both. To build only one,
+pass one of `--platform` or `--sdk` before the `runtimes` argument.
+
+For a faster local build, you can also pass `--no-collect-source-code` and
+`--no-debug-symbols` to skip gathering the source code and generating the debug
+extensions.
+
+Once the runtimes are built, they will be placed in the OSTree repository
+`flatdeb-builddir/ostree-repo`. From there, they can be installed via:
+
+```
+flatpak --user remote-add --no-gpg-verify apertis $PWD/flatdeb-builddir/ostree-repo
+flatpak install apertis org.apertis.{headless,hmi}.{Platform,Sdk}
+```
+
+The sample apps can be built via:
+
+```
+./flatdeb.sh v2022pre amd64 --export-bundles apps apps/org.apertis.headless.hello.yaml
+./flatdeb.sh v2022pre amd64 --export-bundles apps apps/org.apertis.hmi.gnome-font-viewer.yaml
+```
+
+These will result in `.flatpak` bundles being placed inside the
+`flatdeb-builddir` folder.