docs: add guide for creating a new task handler

[harshavardhan194]

Closes #16

Summary

This PR adds documentation to CONTRIBUTING.md describing the process for creating a new task handler.

Changes

• Added guidance for defining a new task
• Documented how to register the task with the CLI (--task)
• Explained how to implement a processing module
• Documented how to add the module to the processing pipeline
• Added instructions for configuring task-specific options
• Included guidance for writing and running unit tests

This documentation is intended to help new contributors implement additional processing tasks consistently..

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[greptile-apps]

Greptile Summary

This PR adds contributor guidance for creating new task handlers. The main changes are:

• New CONTRIBUTING.md section for task-handler setup.
• Checklist covering params, CLI options, graph wiring, and tests.
• Example flow based on the summarization task.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
CONTRIBUTING.md	Adds documentation for implementing and testing new task handlers.

_{Reviews (2): Last reviewed commit: "Merge branch 'main' into docs/task-handl..." | Re-trigger Greptile}