Posting guide: add Android/iOS labels

[hyochan]

Posting guide: categorize Discussions by platform

When creating a new Discussion, select labels to indicate the platform:

• Android — add label android
• iOS — add label ios
• Both — add labels android and ios (or cross-platform)

Quick filters:

• Android: https://github.com/hyodotdev/openiap.dev/discussions?discussions_q=label%3Aandroid
• iOS: https://github.com/hyodotdev/openiap.dev/discussions?discussions_q=label%3Aios
• Both: https://github.com/hyodotdev/openiap.dev/discussions?discussions_q=label%3Across-platform

Thank you!