add README

Author: Adam C. Foltzer

README.md

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+Some handy Template Haskell splices for including the current git hash
+and branch in the code of your project. Useful for including in panic
+messages, `--version` output, or diagnostic info for more informative
+bug reports.
+
+Most of the complication in the `GitRev` module is due to the various
+places the current git hash might be stored:
+
+1. Detached HEAD: the hash is in `.git/HEAD`
+2. On a branch or tag: the hash is in a file pointed to by `.git/HEAD`
+in a location like `.git/refs/heads`
+3. On a branch or tag but in a repository with packed refs: the hash is
+in `.git/packed-refs`
+
+These situations all arise under normal development workflows, but
+there might be further scenarios that cause problems. Let me know if
+you run into them!