Never edit README.md directly. It is generated from README.Rmd (the file header says so: “README.md is generated from README.Rmd”).
If a change should appear in the README, edit README.Rmd and regenerate README.md (e.g. devtools::build_readme()), then commit both files together. A README.md change must never be committed without the corresponding README.Rmd change.
Versioning
Bump the development version once per pull request, not per commit.
The development version lives in the Version: field of DESCRIPTION; it is the fourth, .9000-style component (e.g. 0.5.0.9005 -> 0.5.0.9006). Increment that component by one for the PR.