nix-store: document --add-fixed

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OPERATION --ADD-FIXED
   Synopsis
       nix-store [--recursive] --add-fixed algorithm paths...

   Description
       The operation --add-fixed adds the specified paths to the Nix store. Unlike --add
       paths are registred using the specified hashing algorithm, resulting in the same
       output path as a fixed output derivation. This can be used for sources that are not
       available from a public url or broke since the download expression was written.

       This operation has the following options:

       --recursive
           Use recursive instead of flat hashing mode, used when adding directories
	   to the store.

   Example
           $ nix-store --add-fixed sha256 ./hello-2.10.tar.gz
           /nix/store/3x7dwzq014bblazs7kq20p9hyzz0qh8g-hello-2.10.tar.gz

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