Add organization and documentation to hooks collection

contrib/hooks/encrypt_with_checksums/README.md

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+## Track checksums of encrypted files
+
+Contributed by Martin Zuther
+
+Hook         | Description
+----         | -----------
+post_encrypt | Collects the checksums of encrypted files, and stores them in .config/yadm/files.checksums
+post_list    | Prints the names of encrypted files
+post_status  | Reports untracked changes within encrypted files