Added a read-me.

README.md

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+A small library that examines the internal files of a git repo.
+
+It can handle:
+- objects, stored loose or in packs (including ref and offset deltas)
+- the staging index
+- refs
+- reflogs
+
+Compile the `cli` project (a CLI wrapper around the main library), then run it with `--help` to get help.