[Readme] put badges and links up top

[lrnv]

It would be nice if the different badges (r-cmd-check, licencing, link to the pkgdown docs website) would be at the top of the readme.md, which is their standard location.

Crosslink to openjournals/joss-reviews#9055

[giabaio]

Is that what you had in mind, @lrnv (see current version of the README file)?