📝 Add contribution guidelines to the repository

[lavigarg-simform]

Description

Added a detailed contribution guide for the package.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in doc/ and/or package README(s), and added dartdoc comments with /// where applicable.
• I have updated/added relevant examples in example/ and/or splash_master_{rive,video,lottie}/example/.
• I have updated impacted package metadata/docs (for example: README.md, CHANGELOG.md, and package-level docs where applicable).

Impacted package(s)

• splash_master (core)
• splash_master_rive
• splash_master_video
• splash_master_lottie

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

None

[lavigarg-simform]

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@vatsaltanna-simformsolutions @apurva-simform

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Pull request overview

Adds a new CONTRIBUTING.md document to define how external contributors should report issues and submit pull requests for Splash Master.

Changes:

• Introduces contribution guidance covering bug reports, bug fixes, new features, and documentation changes.
• Adds contributor workflow steps (fork/branch/commit/PR) and a maintainer section for merging and releases.

Comments suppressed due to low confidence (1)

CONTRIBUTING.md:39

• Use the correct "GitHub" capitalization in link text (currently "Github").

- As always first file a [Github issue].