README: Explain what this is for

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+nix-pin-deps: GC-anchor a Nix build's dependencies
+--------------------------------------------------
+
+When a Nix build fails for lack of disk space and a `nix-collect-garbage`
+is needed to make disk space available, `nix-collect-garbage` will
+unhelpfully delete the successfully-built dependencies of the derivation
+that failed because they are are not yet gc-anchored, because the
+build failed.
+
+This tool solves this problem:
+
+  `nix-pin-deps /nix/store/...-foo.drv foo-deps`
+
+will create gc-anchors in foo-deps/ for all of the dependencies of foo.drv
+that exist in the nix store, keeping partial build progress safe from
+`nix-collect-garbage`.