Documentation: Add CONTRIBUTING.md and DEVELOPMENT.md (#158)

As suggested by @Sachin-Bhat per GH-92.

Author: amotl

CONTRIBUTING.md

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+# Contributing to langchain-postgres
+
+This guide is intended to help you get started contributing to langchain-postgres.
+As an open-source project in a rapidly developing field, we are extremely open
+to contributions, whether it be in the form of a new feature, improved infra, or better documentation.
+
+To contribute to this project, please follow the [fork and pull request](https://docs.github.com/en/get-started/quickstart/contributing-to-projects) workflow.
+
+## Reporting bugs or suggesting improvements
+
+Our [GitHub issues](https://github.com/langchain-ai/langchain-postgres/issues) page is kept up to date
+with bugs, improvements, and feature requests. There is a taxonomy of labels to help
+with sorting and discovery of issues of interest. [See this page](https://github.com/langchain-ai/langchain-postgres/labels) for an overview of
+the system we use to tag our issues and pull requests.
+
+If you're looking for help with your code, consider posting a question on the
+[GitHub Discussions board](https://github.com/langchain-ai/langchain/discussions). Please
+understand that we won't be able to provide individual support via email. We
+also believe that help is much more valuable if it's **shared publicly**,
+so that more people can benefit from it.
+
+- **Describing your issue:** Try to provide as many details as possible. What
+  exactly goes wrong? _How_ is it failing? Is there an error?
+  "XY doesn't work" usually isn't that helpful for tracking down problems. Always
+  remember to include the code you ran and if possible, extract only the relevant
+  parts and don't just dump your entire script. This will make it easier for us to
+  reproduce the error.
+
+- **Sharing long blocks of code or logs:** If you need to include long code,
+  logs or tracebacks, you can wrap them in `<details>` and `</details>`. This
+  [collapses the content](https://developer.mozilla.org/en/docs/Web/HTML/Element/details)
+  so it only becomes visible on click, making the issue easier to read and follow.
+
+## Contributing code and documentation
+
+You can develop langchain-postgres locally and contribute to the Project!
+
+See [DEVELOPMENT.md](DEVELOPMENT.md) for instructions on setting up and using a development environment.
+
+## Opening a pull request
+
+Once you wrote and manually tested your change, you can start sending the patch to the main repository.
+
+- Open a new GitHub pull request with the patch against the `main` branch.
+- Ensure the PR title follows semantic commits conventions.
+  - For example, `feat: add new feature`, `fix: correct issue with X`.
+- Ensure the PR description clearly describes the problem and solution. Include the relevant issue number if applicable.

DEVELOPMENT.md

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+# Setting up a Development Environment
+
+This document details how to set up a local development environment that will
+allow you to contribute changes to the project.
+
+Acquire sources and create virtualenv.
+```shell
+git clone https://github.com/langchain-ai/langchain-postgres
+cd langchain-postgres
+uv venv --python=3.13
+source .venv/bin/activate
+```
+
+Install package in editable mode.
+```shell
+poetry install --with dev,test,lint
+```
+
+Start PostgreSQL/PGVector.
+```shell
+docker run --rm -it --name pgvector-container \
+  -e POSTGRES_USER=langchain \
+  -e POSTGRES_PASSWORD=langchain \
+  -e POSTGRES_DB=langchain \
+  -p 6024:5432 pgvector/pgvector:pg16 \
+  postgres -c log_statement=all
+```
+
+Invoke test cases.
+```shell
+pytest -vvv
+```