Start docs directory for developer-oriented documentation

docs/code-overview.md

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+# Tour of the PREreview Codebase
+
+## Key concepts
+
+<dl>
+  <dt>Preprint</dt>
+  <dd>An academic paper prior to formal review and publishing.</dd>
+  <dt>Review</dt>
+  <dd>PREreview's mission is to lower the barrier to entry for academic review of preprints.
+  Reviews can be requested and submitted via the website.</dd>
+</dl>
+
+## Key libraries
+
+* [React](https://reactjs.org/) The UI framework for the PREreview website.
+* [Express.js](https://expressjs.com/) The web framework for the PREreview API; also supplies data to the website.
+
+## Key services
+* [Redis](https://redis.io/) Holds cached results for some operations.
+* [CouchDB](https://couchdb.apache.org/) Primary "source of truth" for PREreview users, reviews, and other application state.
+
+## External services
+PREreview uses well-known preprint servers to find metadata and content for preprints, including:
+
+- arXiv
+- bioArxiv
+- Crossref
+- OpenAire
+- Google Scholar
+- DOI.org
+
+Additionally, PREreview relies on [ORCid](https://orcid.org/) for user authentication.
+
+# API
+Documentation about the API is published as part of the application; https://outbreaksci.prereview.org/api/ .
+This can be edited in `../src/components/api.js`.
+
+# Database Structure
+
+We divide PREreview data among CouchDB collections.
+The design docs for these are stored in `../src/ddocs/`; see [CouchDB documentation](https://docs.couchdb.org/en/stable/ddocs/index.html) for more info on design docs.
+Database queries are consolidated in `../src/db/db.js`.
+
+## `rapid-prereview-docs`
+Stores information about preprints retrieved from authoritative services.
+
+## `rapid-prereview-users`
+Stores information about users who have provided profile information for the site.
+
+## `rapid-prereview-index`
+Stores a partial copy of information from other collections for full-text indexing.