add DEVELOPER.md

Author: gRedHeadphone

DEVELOPER.md

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+# DEVELOPER.md
+
+## Versioning
+
+This library follows [Semantic Versioning](http://semver.org/).
+
+## Processes
+
+### Conventional Commit messages
+
+This repository uses tool [Release Please](https://github.com/googleapis/release-please) to create GitHub and PyPi releases. It does so by parsing your
+git history, looking for [Conventional Commit messages](https://www.conventionalcommits.org/),
+and creating release PRs.
+
+Learn more by reading [How should I write my commits?](https://github.com/googleapis/release-please?tab=readme-ov-file#how-should-i-write-my-commits)
+
+<!-- ## Testing
+
+### Run tests locally
+
+1. Set environment variables for `SPANNER_INSTANCE_ID`, `SPANNER_DATABASE_ID`, `SPANNER_GRAPH_NAME`
+
+1. Run pytest to automatically run all tests:
+
+    ```bash
+    pytest
+    ```
+
+### CI Platform Setup
+
+Cloud Build is used to run tests against Google Cloud resources in test project: langchain-spanner-testing.
+Each test has a corresponding Cloud Build trigger, see [all triggers][triggers].
+These tests are registered as required tests in `.github/sync-repo-settings.yaml`.
+
+#### Trigger Setup
+
+Cloud Build triggers (for Python versions 3.9 to 3.11) were created with the following specs:
+
+```YAML
+name: integration-test-pr-py39
+description: Run integration tests on PR for Python 3.9
+filename: integration.cloudbuild.yaml
+github:
+  name: langchain-google-spanner-python
+  owner: googleapis
+  pullRequest:
+    branch: .*
+    commentControl: COMMENTS_ENABLED_FOR_EXTERNAL_CONTRIBUTORS_ONLY
+ignoredFiles:
+  - docs/**
+  - .kokoro/**
+  - .github/**
+  - "*.md"
+substitutions:
+  _INSTANCE_ID: <ADD_VALUE>
+  _PG_DATABASE: <ADD_VALUE>
+  _GOOGLE_DATABASE: <ADD_VALUE>
+  _VERSION: "3.9"
+```
+
+Use `gcloud builds triggers import --source=trigger.yaml` to create triggers via the command line
+
+#### Project Setup
+
+1. Create an spanner instance and database
+1. Setup Cloud Build triggers (above)
+
+#### Run tests with Cloud Build
+
+* Run integration test:
+
+    ```bash
+    gcloud builds submit --config integration.cloudbuild.yaml --region us-central1 --substitutions=_INSTANCE_ID=$INSTANCE_ID
+    ```
+
+#### Trigger
+
+To run Cloud Build tests on GitHub from external contributors, ie RenovateBot, comment: `/gcbrun`.
+
+#### Code Coverage
+Please make sure your code is fully tested. The Cloud Build integration tests are run with the `pytest-cov` code coverage plugin. They fail for PRs with a code coverage less than the threshold specified in `.coveragerc`.  If your file is inside the main module and should be ignored by code coverage check, add it to the `omit` section of `.coveragerc`.
+
+Check for code coverage report in any Cloud Build integration test log.
+Here is a breakdown of the report:
+- `Stmts`:  lines of executable code (statements).
+- `Miss`: number of lines not covered by tests.
+- `Branch`: branches of executable code (e.g an if-else clause may count as 1 statement but 2 branches; test for both conditions to have both branches covered).
+- `BrPart`: number of branches not covered by tests.
+- `Cover`: average coverage of files.
+- `Missing`: lines that are not covered by tests.
+
+
+[triggers]: https://console.cloud.google.com/cloud-build/triggers?e=13802955&project=langchain-spanner-testing -->