Make document for contract developers using dynamic link

[loloicci]

Information needed

• function names cannot use (e.g. add_event (used by api))

[da1suk8]

The following function names cannot be used as function names for callable_point because they are already used internally.

(Only related to dynamiclink)

• validate_dynamic_link_interface
• add_event
• add_events
• add_attribute
• add_attributes
• get_caller_addr

For example, an error occurs when using the following as a callable_point.

#[callable_point]
fn add_event(deps: DepsMut, _env: Env) {
    ......
}

 = note: rust-lld: error: function signature mismatch: add_event
      ....
      ....