Add minimal docs for the top-level package

Closes restic#173

doc.go

@@ -0,0 +1,11 @@
+// Package restic gives a (very brief) introduction to the structure of source code.
+//
+// Overview
+//
+// The packages are structured so that cmd/ contains the main package for the
+// restic binary, and internal/ contains almost all code in library form. We've
+// chosen to use the internal/ path so that the packages cannot be imported by
+// other programs. This was done on purpose, at the moment restic is a
+// command-line program and not a library. This may be revisited at a later
+// point in time.
+package restic