docs: add mention of file scheme (#2120)

Signed-off-by: Keith Zantow <[email protected]>

README.md

@@ -174,6 +174,7 @@ oci-archive:path/to/yourimage.tar      use a tarball from disk for OCI archives
 oci-dir:path/to/yourimage              read directly from a path on disk for OCI layout directories (from Skopeo or otherwise)
 singularity:path/to/yourimage.sif      read directly from a Singularity Image Format (SIF) container on disk
 dir:path/to/yourproject                read directly from a path on disk (any directory)
+file:path/to/yourfile                  read directly from a file on disk
 sbom:path/to/syft.json                 read Syft JSON from path on disk
 registry:yourrepo/yourimage:tag        pull image directly from a registry (no container runtime required)
 ```