docs: Add basic README

Also add test and coverage badges.

closes #5

chore: Add tests and coverage workflow status badge

The badge should be visible once the workflow ran for the first time.

README.md

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+# RDFProxy
+
+![tests](https://github.com/acdh-oeaw/rdfproxy/actions/workflows/tests.yaml/badge.svg)
+[![coverage](https://coveralls.io/repos/github/acdh-oeaw/rdfproxy/badge.svg?branch=main&kill_cache=1)](https://coveralls.io/github/acdh-oeaw/rdfproxy?branch=main&kill_cache=1)
+[![License: GPL v3](https://img.shields.io/badge/License-GPLv3-blue.svg)](https://www.gnu.org/licenses/gpl-3.0)
+[![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)
+
+A collection of Python utilities for connecting an RDF store to FastAPI.
+
+
+## SPARQLModelAdapter
+
+The `rdfproxy.SPARQLModelAdapter` class allows to run a query against an endpoint and map a flat SPARQL query result set to a potentially nested Pydantic model.
+
+E.g. the result set of the following query
+```sparql
+select ?x ?y ?a ?p
+where {
+    values (?x ?y ?a ?p) {
+        (1 2 "a value" "p value")
+    }
+}
+```
+can be run against an endpoint and mapped to a nested Pydantic model like so:
+
+```python
+from SPARQLWrapper import SPARQLWrapper
+from pydantic import BaseModel
+from rdfproxy import SPARQLModelAdapter, _TModelInstance
+
+class SimpleModel(BaseModel):
+    x: int
+    y: int
+
+class NestedModel(BaseModel):
+    a: str
+    b: SimpleModel
+
+class ComplexModel(BaseModel):
+    p: str
+    q: NestedModel
+
+
+sparql_wrapper = SPARQLWrapper("https://query.wikidata.org/bigdata/namespace/wdq/sparql")
+
+query = """
+select ?x ?y ?a ?p
+where {
+    values (?x ?y ?a ?p) {
+        (1 2 "a value" "p value")
+    }
+}
+"""
+
+adapter = SPARQLModelAdapter(sparql_wrapper=sparql_wrapper)
+models: list[_TModelInstance] = adapter(query=query, model_constructor=ComplexModel)
+```
+
+This produces an Iterable of Pydantic model instances (in the above case: `[ComplexModel(p='p value', q=NestedModel(a='a value', b=SimpleModel(x=1, y=2)))]`) which can then be served via FastAPI.
+
+The `model_constructor` parameter takes either a Pydantic model directly or a model_constructor callable which receives the raw `SPARQLWrapper.QueryResult` object and is responsible for returning an Iterable of model instances.
+