rework readme

[shajoezhu]

Reporting an Issue with rtables

Do not use this form to ask a question, or ask for assistance. Instead, ask on Stackoverflow using the nest-rtables tag. Questions about a function's use will be closed without a response.

Please briefly describe your problem and, when relevant, the output you expect.
Please also provide the output of utils::sessionInfo() or
devtools::session_info() at the end of your post.

If at all possible, please include a minimal, reproducible
example.
The rtables team will be much more likely to resolve your issue if they are
able to reproduce it themselves locally.

Please delete this preamble after you have read it.

---

your brief description of the problem

library(rtables)

# your reproducible example here