Create module-taxonomy.md

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+# Module Taxonomy
+
+Marginalia has a lot of modules, and their names have specific meanings with architectural consequences. These are outlined below.
+
+## Library
+
+A library is independent of the search engine domain. It solves a single problem. Maybe it's a B-Tree implementation, 
+a locality sensitive hash algorithm. Whatever. It does not know what an URL is, or a document. It's more primitive 
+than that.
+
+These could hypothetically be broken off and shipped separately, or just yoinked from the codebase and used elsewhere. 
+These libraries are co-licensed under MIT to facilitate that, the rest of the search engine is and will be AGPL.
+
+## Feature
+
+A feature is essentially a domain-specific library. It solves some specific problem. Maybe extracting keywords from a 
+document, or parsing a search query. Features exist to separate conceptually isolated logic. It may only depend on 
+libraries and models.
+
+## Models
+
+A module package contains domain-specific data representations. It may contain light logic related to e.g. serialization, but should
+primarily focus on the data.
+
+## APIs
+
+A module package contains domain-specific interface between processes.
+
+## Process
+
+A process is a batch job that reads files and performs some task. It may depend on libraries, features and models. It may not explicitly 
+depend on a service or another process.
+
+## Service
+
+A service offers a web service interface. It may depend on libraries, features and models and APIs. It may not explicitly depend on a 
+process or another service.