Add CONTRIBUTING.md

Author: Schultzer

CONTRIBUTING.md

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+<!--
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: 2025 DBVisor
+-->
+
+# Contributing to SQL
+
+We invite contributions to SQL, in order for you to commit with confindance 
+you can create the database with `mix sql.create` 
+and explore the library in `iex -S mix`. 
+
+For conformance testing you'll need to clone https://github.com/elliotchance/sqltest 
+and run `mix sql.gen.test ../sqltest/standards/2016`.
+
+You can run benchmarks with `mix sql.bench`, remember that performance can differ 
+based on concurrency.
+
+As we're pre 1.0, then breaking changes are allowed, although they will only be merged, if
+they improve: userability, correctness and performance.