README: document required dependency

As a dependency of `images` we also need `gpgme-devel` here,
if we compile without `containers_image_openpgp`.

README.md

@@ -32,6 +32,21 @@ $ go install github.com/osbuild/image-builder-cli/cmd/image-builder@main
 
 We plan to provide rpm packages in fedora as well.
 
+## Compilation
+
+You can compile the application in `cmd/image-builder` with
+the normal `go` command or use
+
+```
+make build
+```
+
+To compile without go build tags you will need to install
+the required RPMs:
+
+```console
+sudo dnf install gpgme-devel
+```
 
 ## Prerequisites