Add start of developer's intro

docs/dev_intro.md

@@ -0,0 +1,45 @@
+# Developer's Introduction
+
+## Overview 
+
+ContentDB is a Python Flask webservice, with a PostgreSQL database.
+
+To avoid blocking web requests, background jobs run as
+[Celery](https://docs.celeryproject.org/en/stable/getting-started/introduction.html) tasks.
+
+
+## Locations
+
+### The App
+
+The `app` directory contains the Python Flask application.
+
+* `blueprints` contains all the Python code behind each endpoint. 
+    A [blueprint](https://flask.palletsprojects.com/en/2.0.x/blueprints/) is a Flask construct to hold a set of endpoints.
+* `templates` contains all the HTML templates used to generate responses. Each directory in here matches a director in blueprints.
+* `models` contains all the Database table classes. ContentDB uses [SQLAlchemy](https://docs.sqlalchemy.org/en/14/) to interact with PostgreSQL.
+* `flatpages` contains all the markdown user documentation, including `/help`.
+* `public` contains files that should be added to the web server unedited. Examples include CSS libraries, images, and JS scripts.
+* `scss` contains the stylesheet files, that are compiled into CSS.
+* `tasks` contains the background tasks executed by [Celery](https://docs.celeryproject.org/en/stable/getting-started/introduction.html).
+* `logic` is a collection of reusable functions. For example, shared code to create a release or edit a package is here.
+* `tests` contains the Unit Tests and UI tests.
+* `utils` contain generic Python utilities, for example common code to manage Flask requests.
+
+There are also a number of Python files in the `app` directory. The most important one is `querybuilder.py`,
+which is used to generate SQLAlachemy queries for packages and topics.
+
+### Supporting directories
+
+* `migrations` contains code to manage database updates.
+* `translations` contains user-maintained translations / locales.
+* `utils` contains bash scripts to aid development and deployment.
+
+
+## How to find stuff
+
+Generally, you want to start by finding the endpoint and then seeing the code it calls.
+
+Endpoints are sensibly organised in `app/blueprints`. 
+
+You can also use a file search. For example, to find the package edit endpoint, search for `.route("/packages/<author>/<name>/edit/")`.