Added CONTRIBUTING.md

Author: inelpandzic

CONTRIBUTING.md

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+# Contributing guide
+
+Welcome to Percona ClusterSync for MongoDB!
+
+1. [Prerequisites](#prerequisites)
+2. [Submitting a pull request](#submitting-a-pull-request)
+3. [Building Percona ClusterSync for MongoDB](#building-percona-clustersync-for-mongodb)
+4. [Running PCSM locally](#running-pcsm-locally)
+5. [Running tests locally](#running-tests-locally)
+6. [Contributing to documentation](#contributing-to-documentation)
+
+We're glad that you would like to become a Percona community member and participate in keeping open source open.
+
+Percona ClusterSync for MongoDB (PCSM) is a tool for cloning and replicating data between MongoDB clusters. It supports both replica sets and sharded clusters, handling initial data cloning followed by continuous change replication.
+
+You can contribute in one of the following ways:
+
+1. Reach us on our [Forums](https://forums.percona.com/c/mongodb/percona-clustersync-for-mongodb).
+2. [Submit a bug report or a feature request](https://jira.percona.com/projects/PCSM)
+3. Submit a pull request (PR) with the code patch
+4. Contribute to documentation
+
+## Prerequisites
+
+Before submitting code contributions, we ask you to complete the following prerequisites.
+
+### 1. Sign the CLA
+
+Before you can contribute, we kindly ask you to sign our [Contributor License Agreement](https://cla-assistant.percona.com/percona/percona-clustersync-mongodb) (CLA). You can do this in one click using your GitHub account.
+
+**Note**: You can sign it later, when submitting your first pull request. The CLA assistant validates the PR and asks you to sign the CLA to proceed.
+
+### 2. Code of Conduct
+
+Please make sure to read and agree to our [Code of Conduct](https://github.com/percona/community/blob/main/content/contribute/coc.md).
+
+## Submitting a pull request
+
+All bug reports, enhancements and feature requests are tracked in [Jira issue tracker](https://jira.percona.com/projects/PCSM). Though not mandatory, we encourage you to first check for a bug report among Jira issues and in the PR list: perhaps the bug has already been addressed.
+
+For feature requests and enhancements, we do ask you to create a Jira issue, describe your idea and discuss the design with us. This way we align your ideas with our vision for the product development.
+
+If the bug hasn't been reported / addressed, or we've agreed on the enhancement implementation with you, do the following:
+
+1. [Fork](https://docs.github.com/en/github/getting-started-with-github/fork-a-repo) this repository
+2. Clone this repository on your machine
+3. Create a separate branch for your changes. If you work on a Jira issue, please include the issue number in the branch name so it reads as `pcsm-XXX-my-branch`. This makes it easier to track your contribution.
+4. Make your changes. Please follow the guidelines outlined in the [Go Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments) to improve code readability.
+5. Test your changes locally. See the [Running tests locally](#running-tests-locally) section for more information
+6. Commit the changes. Add the Jira issue number at the beginning of your message subject so that it reads as `PCSM-XXX - My subject`. The [commit message guidelines](https://gist.github.com/robertpainsi/b632364184e70900af4ab688decf6f53) will help you with writing great commit messages
+7. Open a PR to Percona
+8. Our team will review your code and if everything is correct, will merge it. Otherwise, we will contact you for additional information or with the request to make changes.
+
+## Building Percona ClusterSync for MongoDB
+
+To build Percona ClusterSync for MongoDB from source code, you require the following:
+
+* Go 1.25 or above. See [Installing and setting up Go tools](https://golang.org/doc/install) for more information
+* make
+
+To build the project, run the following commands:
+
+```sh
+git clone https://github.com/<your_name>/percona-clustersync-mongodb
+cd percona-clustersync-mongodb
+make build
+```
+
+After `make` completes, you can find the `pcsm` binary in the `./bin` directory:
+
+```sh
+./bin/pcsm version
+```
+
+By running `pcsm version`, you can verify if Percona ClusterSync for MongoDB has been built correctly and is ready for use.
+
+For development with race detection and debugging enabled, use:
+
+```sh
+make test-build
+```
+
+## Running PCSM locally
+
+To run PCSM locally for development and testing, follow these steps:
+
+### 1. Deploy MongoDB clusters
+
+Start the source and target sharded clusters using Docker Compose:
+
+```sh
+MONGO_VERSION="8.0" ./hack/sh/run.sh
+```
+
+For replica set clusters instead of sharded:
+
+```sh
+MONGO_VERSION="8.0" ./hack/rs/run.sh
+```
+
+### 2. Update /etc/hosts
+
+Add the following entries to your `/etc/hosts` file to resolve the container hostnames:
+
+```
+# pcsm test clusters
+127.0.0.1 rs00 rs01 rs02 rs10 rs11 rs12
+127.0.0.1 src-rs00 src-rs10 src-cfg0 src-mongos
+127.0.0.1 tgt-rs00 tgt-rs10 tgt-cfg0 tgt-mongos
+```
+
+### 3. Start PCSM
+
+Run PCSM connecting to the source and target clusters:
+
+```sh
+make pcsm-start SOURCE="mongodb://src-mongos:27017" TARGET="mongodb://tgt-mongos:29017"
+```
+
+## Running tests locally
+
+When you work, you should periodically run tests to check that your changes don't break existing code.
+
+### Go Unit Tests
+
+Run all unit tests with race detection:
+
+```sh
+go test -race ./...
+```
+
+Or using make:
+
+```sh
+make test
+```
+
+To run a single test:
+
+```sh
+go test -race -run TestName ./package
+```
+
+For example:
+
+```sh
+go test -race -run TestFilter ./sel
+go test -race -v -run TestSanitizeConnString ./topo
+```
+
+### Python E2E Tests
+
+Python E2E tests require MongoDB clusters to be running and PCSM to be available.
+
+#### 1. Deploy MongoDB clusters and run PCSM
+
+Follow the steps in [Running PCSM locally](#running-pcsm-locally).
+
+#### 2. Set environment variables
+
+```sh
+export TEST_SOURCE_URI="mongodb://src-mongos:27017"
+export TEST_TARGET_URI="mongodb://tgt-mongos:29017"
+export TEST_PCSM_URL="http://localhost:2242"
+```
+
+You don't have to run PCSM manualy, you can let pytest to automatically manage PCSM process. Just export this:
+
+```sh
+export TEST_PCSM_BIN="./bin/pcsm"
+```
+
+#### 3. Run tests
+
+Run all E2E tests:
+
+```sh
+poetry run pytest
+```
+
+Run a single test:
+
+```sh
+poetry run pytest tests/test_collections.py::test_clone_collection
+```
+
+Run tests including slow tests (disabled by default):
+
+```sh
+poetry run pytest --runslow
+```
+
+## Contributing to documentation
+
+We welcome contributions to our documentation.
+
+Documentation source files are in the [dedicated docs repository](https://github.com/percona/pcsm-docs).
+
+Please follow the contributing guidelines in that repository for how to contribute to documentation.
+
+## After your pull request is merged
+
+Once your pull request is merged, you are an official Percona Community Contributor. Welcome to the community!