Add CONTRIBUTING guide section explaining how to add a new `ContractError` variant

[Kingsman-99]

Description

CONTRIBUTING.md does not explain how to add a new error variant to ContractError in contracts/split/src/error.rs. New contributors often append variants without checking the stability note ("Discriminants are stable — never reorder, only append"). A dedicated section in CONTRIBUTING.md would prevent breaking changes.

Acceptance Criteria

• Add a "Adding a new ContractError variant" section to CONTRIBUTING.md
• The section explains: always append (never reorder), discriminant assignment, updating docs
• Include a minimal before/after code snippet showing the correct way to add a new variant
• cargo test passes (no code changes, only docs)

Context

• Target file: CONTRIBUTING.md
• Reference: contracts/split/src/error.rs — stability comment on line 3

[0xApana]

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Hi, maintainer i'll like to work on this.

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[laddyr141-ui]

@laddyr141-ui has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

i would love to work on this

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[chainsmt]

@chainsmt has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello! Can I work on this issue please?

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