add readme to model-engine folder (#220)

Author: phil-scale

model-engine/README.md

@@ -0,0 +1,43 @@
+# Model Engine
+
+The Model Engine is an API server that allows users to create, deploy, edit,
+and delete machine learning endpoints. It consists of two main architectural
+components:
+
+- The [gateway](./model_engine_server/entrypoints/start_fastapi_server.py)
+  provides a REST API for users to interact with. The routes of the REST API are
+  defined in [`model_engine_server.api`](./model_engine_server/api).
+- The [`model_engine_server.service_builder`](./model_engine_server/service_builder)
+  package is the part of the code that creates the inference pods. It is the
+  endpoint builder. When we do a `POST` request to `/endpoints`, this gets run.
+  It gets run when users create or edit endpoints with `[POST,PUT] /v1/model-endpoints`
+
+There are two other microservices:
+
+- The [kubernetes cache](./model_engine_server/entrypoints/k8s_cache.py)
+  stores endpoint metadata on Redis so that Model Engine does not overload the API
+  server.
+- The celery autoscaler (link TBD) automatically scales
+  the number of inference pods based on the number of requests for async endpoints.
+
+## Getting started
+
+Be sure to install the global `../requirements-dev.txt` first prior
+to any installations of requirements in this directory
+(`pip install -r ../requirements-dev.txt`), as well as the pre-commit hooks
+(`pre-commit install` in the `llm-engine` root folder). Then, install the
+requirements files and this folder as editable
+
+```bash
+pip install -r requirements.txt && \
+    pip install -r requirements-test.txt && \
+    pip install -r requirements_override.txt && \
+    pip install -e .
+```
+
+Run `mypy . --install-types` to set up mypy.
+
+## Testing
+
+Most of the business logic in Model Engine should contain unit tests, located in
+[`tests/unit`](./tests/unit). To run the tests, run `pytest`.