Improve the README for both GitHub and docs.rs

[breezewish]

There are some typical usages of a tracing library. One may be interested in the following examples (comments are welcome!):

• [ ] Basic (multiple function spans) - an implicit style can be involed.
• [ ] Macro shortcut - use macros to make life easier.
• [ ] Async - We are in year 2022 now. Async is the new standard.
• [ ] Send spans across threads - This is less common compared to async in year 2022.
• [ ] Other Advanced Usages

[andylokandy]

Maybe the content in lib.rs has fulfilled the topics you mentioned: https://docs.rs/minitrace/0.4.0/minitrace/