README: Document some details about checks

README.md

@@ -4,6 +4,25 @@ flatpak-builder-lint is a linter for flatpak-builder manifests, and more widely,
 also Flatpak builds. It is primarily developed for Flathub, but can be useful
 for other Flatpak repositories.
 
+## Checks
+
+There are four checks available right now - the `manifest` check, the
+`builddir` check, the `repo` check and the `appstream` check.
+
+- The `manifest` check expects a [Flatpak manifest](https://docs.flatpak.org/en/latest/manifests.html)
+  as input.
+- The `appstream` check expects a [Metainfo file](https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#path-and-filename)
+  as input.
+- The `builddir` check expects a build directory generated by
+  Flatpak Builder as input.
+- The `repo` check expects an OSTree repository generated by Flatpak
+  Builder as input.
+
+The last two are created when [building the application](https://docs.flathub.org/docs/for-app-authors/submission#build-and-install)
+with the proper arguments to Flatpak Builder.
+
+Some checks may require network connectivity.
+
 ## Installation
 
 The only supported ways to install and use it are Flatpak and Docker.