Make badges section a paragraph that pkgdown can recognize

[maelle]

For pkgdown to recognize the badges paragraph and put it in the sidebar as "Dev status" (see e.g. https://docs.ropensci.org/ropenaq/) it needs to be a paragraph, and to be even more certain it'll be recognized I've added the badges paragraph limits that pkgdown uses when the badges paragraph isn't the first paragraph (although here it's the first paragraph).

But maybe you don't want that sidebar thing. :-)

[codecov-io]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #43   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines         729    729           
=====================================
  Hits          729    729

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[ThierryO]

Thanks