docs: rewrite README for third-party developers

CONTRIBUTING.md

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+# Contributing
+
+Everything under `sdk/` is generated. Do not edit it by hand, because the next
+regeneration will overwrite your changes. Fix the OpenAPI spec instead, then
+regenerate.
+
+## Regenerating the SDK
+
+The client is produced by [Kiota](https://learn.microsoft.com/openapi/kiota/)
+1.34.1 from the upstream Rixl OpenAPI spec. With the `kiota` CLI on your PATH:
+
+```bash
+./gen.sh
+```
+
+That runs:
+
+```bash
+kiota generate \
+    -l java \
+    -c RixlClient \
+    -n com.rixl.sdk \
+    -d https://raw.githubusercontent.com/rixlhq/openapi/refs/heads/main/openapi.yaml \
+    -o "./sdk" \
+    --clean-output \
+    --exclude-backward-compatible
+```
+
+`--clean-output` wipes `sdk/` first, so regeneration is the only way changes to
+the spec reach this repo. Commit the result on its own, without mixing in
+hand-written changes.
+
+## Building
+
+```bash
+mvn -q compile
+```
+
+The build compiles `sdk/` directly (`sourceDirectory` in `pom.xml`) and targets
+Java 17.
+
+## Releasing
+
+Releases are cut by release-please from conventional commits. The version lives
+in `pom.xml` and `.release-please-manifest.json`, and both are updated by the
+release PR, so do not bump them by hand. When the version changes, update the
+install snippets in `README.md` to match.