Add CONTRIBUTING.md to sdk module

sdk/CONTRIBUTING.md

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+# Contributing to go.opentelemetry.io/auto/sdk
+
+The `go.opentelemetry.io/auto/sdk` module is a purpose built OpenTelemetry SDK.
+It is designed to be:
+
+0. An OpenTelemetry compliant SDK
+1. Instrumented by auto-instrumentation (serializable into OTLP JSON)
+2. Lightweight
+3. User-friendly
+
+These design choices are listed in the order of their importance.
+
+The primary design goal of this module is to be an OpenTelemetry SDK.
+This means that it needs to implement the Go APIs found in `go.opentelemetry.io/otel`.
+
+Having met the requirement of SDK compliance, this module needs to provide code that the `go.opentelemetry.io/auto` module can instrument.
+The chosen approach to meet this goal is to ensure the telemetry from the SDK is serializable into JSON encoded OTLP.
+This ensures then that the serialized form is compatible with other OpenTelemetry systems, and the auto-instrumentation can use these systems to deserialize any telemetry it is sent.
+
+Outside of these first two goals, the intended use becomes relevant.
+This package is intended to be used in the `go.opentelemetry.io/otel` global API as a default when the auto-instrumentation is running.
+Because of this, this package needs to not add unnecessary dependencies to that API.
+Ideally, it adds none.
+It also needs to operate efficiently.
+
+Finally, this module is designed to be user-friendly to Go development.
+It hides complexity in order to provide simpler APIs when the previous goals can all still be met.