Instructions in README

[langtonhugh]

Currently the repo's README file does not have any instructions for first time codecheckers. Things that might be useful:

• Guidelines on whether/when to fork the researcher's repository to the codechecker organisation (or not).
• Where to execute codecheck::create_codecheck_files() in the file structure.
• Definitions of things like 'manifest files'.
• As per #58 , making the template RMD usable.

[nuest]

In lack of another better documentation, we could also reference the community process documentation at https://codecheck.org.uk/guide/community-workflow That document is currently written independent of the R package.

@langtonhugh Would you be fine with focusing the content in the README on using the R package for a check? One sentence on what the manifest entails is definitely a good idea.

I can in parallel try to improve other documents to better include the content that better fits elsewhere/that we would like to reference from the README.