doc(back-end): update README

[AdrieneTZ]

Add:

• Swagger UI link
• Some descriptions of snowball
• Tech Stack of back-end
• APIs, including the argument type in requests and responses, and detailed response errors

[vercel[bot]]

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1 Ignored Deployment

| Name | Status | Preview | Updated | | :--- | :----- | :------ | :------ | | **snowball** | ⬜️ Ignored ([Inspect](https://vercel.com/davipon/snowball/5esXsvuRKpzSx6i5cmSLDi3JBMvY)) | | Nov 24, 2022 at 0:54AM (UTC) |