docs(ISSUE-1325): Add Identifier Conventions

[toastsec]

What does this change?

ISSUE-1325 ( #1325 ) identified that Identifier conventions were not documented. This change modifies the contributing.md file to include the following information

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Identifier Conventions

All submissions related to the use of different identifiers should adhere to the following conventions.

Identifier	Notes	Example
SHA-256	lowercase	801d1dd8bc78984c126a269aca053642d16eef4389dfdc8df575af929fdcf279
CDHash	lowercase	2d1cff4b1080058e7e5913e5a3398bcd0199b6a4
TeamID	uppercase	43AQ936H96
Signing ID	case insensitive	EQHXZ8M8AV:com.google.Chrome

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[mlw]

Thank you!