README - join up with dfeR::create_project() and update to be .md files

[cjrace]

Currently at https://dfe-analytical-services.github.io/analysts-guide/RAP/rap-statistics.html#writing-a-readme-file, we have this:

We should update it a bit as it's not strictly accurate anymore:

• [ ] Switch .txt to .md, and point to some basic markdown guidance as well. Explain / show how .md render nicely in Azure DevOps / GitHub etc
• [ ] Point to the dfeR::create_project() function for how to get started
• [ ] Join up with with the dfeR function / dfeR package more widely so there is a template readme and link to that

[Lsnaathorst1]

README improved guidance & example