feat(docs): Setup Documentation (#6907)

* Start adding documentation.

* Update README.md

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Co-authored-by: Matthew Slipper <me@matthewslipper.com>

docs/README.md

+## Optimism Monorepo Documentation
+
+The `docs/` directory contains Optimism documentation closely tied to the implementation details of the monorepo (https://github.com/ethereum-optimism/optimism).
+
+The directory layout is divided into the following sub-directories.
+
+- [`postmortems/`](./postmortems/): Timestamped post-mortem documents.
+- [`security-reviews`](./security-reviews/): Audit summaries and other security review documents.