Adds contributing guide (#112)

Author: jacoblee93

CONTRIBUTING.md

@@ -0,0 +1,122 @@
+# Contributing
+
+Thanks for your interest in contributing to the LangSmith Java SDK!
+
+## Overview
+
+This SDK is **auto-generated by [Stainless](https://www.stainless.com/)** from our OpenAPI spec. Custom code (prompts, OTel wrappers, etc.) lives alongside generated code — Stainless is configured to [preserve custom code](https://www.stainless.com/docs/sdks/configure/custom-code/) and never overwrite it.
+
+> **Important:** Most files in the SDK are generated. If you need to change generated code, talk to @edongashi — there may be weird errors in the Stainless config, but they're generally fixable.
+
+## Branching and workflow
+
+| Branch                                          | Purpose                                               |
+| ----------------------------------------------- | ----------------------------------------------------- |
+| `next`                                          | **Development branch.** All PRs should target `next`. |
+| `main`                                          | Release branch. Only updated via release-please.      |
+| `release-please--branches--main--changes--next` | Auto-generated release PR. Merges `next` → `main`.    |
+
+**Always work off `next`.** Do not merge directly to `main` — Dependabot or other bots occasionally violate this, which can cause headaches.
+
+### Release process
+
+1. Merge your PR into `next`.
+2. A release-please PR is automatically created/updated (`release-please--branches--main--changes--next`).
+3. Verify your change appears in the release PR diff and description.
+4. Approve the release PR — it auto-merges on approval.
+5. A GitHub Action publishes to Maven Central via `./gradlew publishAndReleaseToMavenCentral`.
+6. Maven Central can be slow to index — verify on [repo1.maven.org](https://repo1.maven.org/maven2/com/langchain/smith/langsmith-java/) rather than [mvnrepository.com](https://mvnrepository.com/artifact/com.langchain.smith/langsmith-java), which lags behind.
+
+> **Note:** If lint fixes or other changes are pushed to the release-please branch after a version bump, the publish action can fail. Always ensure CI is green before approving.
+
+## Development setup
+
+### Prerequisites
+
+- Java 8+ (tested against 8 and 21)
+- Gradle (uses the included `gradlew` wrapper)
+
+### Building
+
+```bash
+./scripts/build
+```
+
+### Formatting
+
+The project uses [ktfmt](https://github.com/facebook/ktfmt) (`--kotlinlang-style`) for Kotlin and [Palantir Java Format](https://github.com/palantir/palantir-java-format) for Java.
+
+```bash
+# Format all files
+./scripts/format
+
+# Format only Kotlin
+./gradlew formatKotlin
+
+# Format a single module
+./gradlew :langsmith-java-core:formatKotlin
+```
+
+### Linting
+
+```bash
+# Lint all files
+./scripts/lint
+
+# Lint only Kotlin
+./gradlew lintKotlin
+```
+
+CI blocks on lint failures. Always run `./scripts/format` before pushing.
+
+### Testing
+
+```bash
+# Run all tests
+./scripts/test
+
+# Run specific tests
+./gradlew :langsmith-java-core:test --tests "com.langchain.smith.prompts.*"
+
+# Force re-run (skip Gradle cache)
+./gradlew :langsmith-java-core:test --tests "..." --rerun
+
+# See stdout output
+./gradlew :langsmith-java-core:test --tests "..." --rerun --info
+```
+
+#### Integration tests
+
+Integration tests require API keys and skip gracefully via `assumeTrue` if they're missing:
+
+```bash
+export LANGSMITH_API_KEY="lsv2_pt_..."
+export OPENAI_API_KEY="sk-..."
+export ANTHROPIC_API_KEY="sk-ant-..."
+```
+
+## CI
+
+CI runs three jobs on every push and PR:
+
+1. **lint** — `./scripts/lint`
+2. **build** — `./scripts/build`
+3. **test** — `./scripts/test`
+
+All three must pass before merging.
+
+## Code style
+
+See [AGENTS.md](AGENTS.md) for detailed Kotlin conventions, including:
+
+- Prefer `buildMap`/`buildList` over mutable collections
+- Use `partition` instead of double `filter`
+- Use parameterized tests for table-driven cases
+- Avoid `@Suppress("UNCHECKED_CAST")` — restructure with safe casts
+- `toString()` should follow the `ClassName{field=value}` convention
+
+## Dependencies
+
+- **Anthropic SDK** is a `compileOnly` dependency — users must add it themselves. Methods using Anthropic types should catch `NoClassDefFoundError` and throw `IllegalStateException` with a clear message.
+- **SLF4J** (`slf4j-api`) is an `api` dependency for logging. Tests use `slf4j-simple`.
+- **Jackson** 2.18.2 is the default, but the SDK is compatible with 2.13.4+. Tests compile against 2.14.0.