docs: add architecture description

ARCHITECTURE.md

@@ -1,3 +1,53 @@
 # Architecture
 
+This document describes the high-level architecture of PostgREST.
+
+## Bird's Eye View
+
 ![PostgREST Architecture](docs/_static/arch.png)
+
+On the highest level, PostgREST processes the user HTTP request, if it's accepted it generates a SQL query, executes it, and produces a response.
+
+## Code Map
+
+This section talks briefly about various important modules.
+
+The starting points of the program are `main/Main.hs` -> `src/PostgREST/CLI.hs` -> `src/PostgREST/App.hs`.
+
+`App.hs` is then in charge of composing the different modules.
+
+### Auth
+
+It authenticates the request using JWT.
+
+### ApiRequest
+
+PostgREST operates over two types of resources: database relations (tables or views) and database functions; providing different representations (depending on the media type)
+for them.
+
+This module is in charge of representing the operation over an `ApiRequest` type. It parses the URL querystring following PostgREST syntax, the request headers, and the request body
+(if possible it avoids parsing the body and sends it directly to the db).
+
+A request might be rejected at this level if it's invalid. For example when providing an unknown media type to PostgREST or using an unknown HTTP method.
+
+### Plan
+
+Using the Schema Cache, this module enables more complex functionality (like resource embedding) by enriching the ApiRequest. It generates intermediate Plan types
+that then will be used for the next step.
+
+A request might be rejected at this level if it's invalid. For example when doing resource embedding on a nonexistent resource.
+
+### Query
+
+This module generates SQL queries that are parametrized and prepared. Only at this stage a connection from the pool might be used.
+
+A query might fail (and be rollbacked) at this level if it doesn't comply to certain conditions, e.g. by not returning a single row when a ``Accept: application/vnd.pgrst.object``
+header is specified.
+
+### SchemaCache
+
+This queries the PostgreSQL system catalogs and caches the results into a SchemaCache type, which then is used by the other modules.
+
+### Admin
+
+Admin server exposed in another port, it provides health checks.