Add contributing guidelines to the project

[ankitaggarwal011]

This will make things easier for the contributors.

Some important points to be highlighted in the contributing guide would be:

• How to contribute - improve documentation/add new methods/improve performance.
• Explain contribution workflow - issues/pull-request/review.
• An overview of code structure and API.
• Coding practices to follow.
• Guide to add a new image processing method.

Any suggestions are welcome. Thanks.

[Subway19]

Looks interesting.

[iabhi7]

Doing it 👍
The overview is already in the README file so I guess we can leave it in the contribution.md?

[ankitaggarwal011]

Thank you for your interest @vibrantabhi19. I appreciate it. @skinnejj was already working on this in #16. Please take a look at it. This issue can have multiple contributors. Thank you.

Edit: It might be better to document the overview of code structure and API can be written in a separate document rather than this. You can leave that section at this time. Thank you.