Add minimal documentation for backend/{local,sftp}

backend/local/doc.go

@@ -0,0 +1,2 @@
+// Package local implements repository storage in a local directory.
+package local

backend/sftp/doc.go

@@ -0,0 +1,3 @@
+// Package sftp implements repository storage in a directory on a remote server
+// via the sftp protocol.
+package sftp