Add documentation for backend development

[lunny]

To clarify how the Gitea backend code organized, I propose the document to help contributors on writing backend codes.

[wxiaoguang]

Some more things:

• singular or not, eg: models/user vs models/users, models/issue vs models/issues
• import alias, camelCase or snake_case

[lunny]

Some more things:

• singular or not, eg: models/user vs models/users, models/issue vs models/issues
• import alias, camelCase or snake_case

• I like singular
• I think snake_case is better to differ from variables.

[KN4CK3R]

I have a problem with singular in my package PR because package is a reserved word in Go, so I used packages. So I don't know if we can enforce this.

[wxiaoguang]

Some more things:

• singular or not, eg: models/user vs models/users, models/issue vs models/issues
• import alias, camelCase or snake_case

* I like singular

* I think snake_case is better to differ from variables.

Both are fine to me. For singular, it's more likely to conflict with normal variable names. For example, usually we have alot of code like this user := GetUser(), then user.Xxx conflicts.

Since we can make an agreement, these points can be written into document, including how to name variables to prevent conflict, then we have a more clear review guideline.

[lunny]

• singular

I'm ok for some special situations.

[wxiaoguang]

LGTM, More points:

1. db.WithTx seems easier than db.TxContext
2. About error tracing (shared in the discord channel)
   • https://goteleport.com/blog/golang-error-handling/
   • https://github.com/gravitational/trace/
3. A general method to do safe UPSERT

[lunny]

LGTM, More points:

1. db.WithTx seems easier than db.TxContext

2. About error tracing (shared in the discord channel)

   • https://goteleport.com/blog/golang-error-handling/
   • https://github.com/gravitational/trace/

3. A general method to do safe UPSERT

About the tracing, could you help to open an issue to describe the idea? And also if there is an example PR, that's great.