docs: add trpc-go framework configuration

[WineChord]

No description provided.

[WineChord]

主库的 config 目录下的 README 放了业务配置相关的内容，框架配置就暂时放到 docs 中了

[liuzengh]

This requirement shouldn't be mandatory, right?

// ExpandEnv looks for ${var} in s and replaces them with value of the corresponding environment variable.
// It's not like os.ExpandEnv which handles both ${var} and $var.
// $var is considered invalid, since configurations like password for redis/mysql may contain $.
func ExpandEnv(s []byte) []byte {
// ...
}

[WineChord]

这个意思大概说的是 ExpandEnv 只查 ${} 不查 $，所以 password 严格意义上来讲应该是不能包含 ${xx} 这种形式