Add DCO (Signed-off-by) requirement in contributing guidelines

# Task description

While creating PRs, some commits are failing due to missing Signed-off-by line (DCO check).

Currently, this is not clearly mentioned in the contributing guidelines, so new contributors may face confusion.

We need to update the contributing guide to include DCO (Developer Certificate of Origin) requirement.

# Solution

Add a small section in the contributing guidelines (under commit message or PR section):

- Explain that every commit must include:

`Signed-off-by: Your Name `

- Mention using:

`git commit -s`

to automatically add sign-off

- (Optional) Add a short note on how to fix missing sign-offs using rebase.

# Who can join or take the task

The good first issue is intended for first-time contributors to get started on his/her contributor journey.

After a contributor has successfully completed 1-2 good first issue's, they should be ready to move on to help wanted items, saving the remaining good first issue for other new contributors.

# How to join or take the task

Just reply on the issue with the message `/assign` in a separate line.

Then, the issue will be assigned to you.

# How to ask for help

If you need help or have questions, please feel free to ask on this issue.

The issue author or other members of the community will guide you through the contribution process.