Add contributing

CONTRIBUTING.md

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+# CONTRIBUTING
+
+## Fixing typos
+
+Small typos or grammatical errors in documentation may be edited directly using
+the GitHub web interface, so long as the changes are made in the *source* file.
+
+-   YES ✔️: you edit a roxygen comment in a `.R` file below `R/`.
+-   NO ❌: you edit an `.Rd` file below `man/`.
+
+## Prerequisites
+
+Before you make a substantial pull request, you should always file an issue and
+make sure someone from the team agrees that it's a problem. If you've found a
+bug, create an associated issue and illustrate the bug with a minimal
+[reprex](https://www.tidyverse.org/help/#reprex).
+
+## Pull request process
+
+-   We recommend that you create a Git branch for each pull request (PR).
+-   Look at the `R-CMD-check` build status before. After making changes, run
+    `devtools::check()` and ensure no new Notes, Warnings or Errors are
+    appearing.
+-   We recommend the tidyverse [style guide](http://style.tidyverse.org). We
+    also use the [styler](https://CRAN.R-project.org/package=styler) package to
+    apply these styles, but please **don't restyle code that has nothing to do
+    with your PR**.
+-   We use [roxygen2](https://cran.r-project.org/package=roxygen2).
+-   We use [testthat](https://cran.r-project.org/package=testthat).
+    Contributions with test cases included are easier to accept.
+-   For user-facing changes, add a bullet to the top of `NEWS.md` below the
+    current development version header describing the changes made followed by
+    your GitHub username, and links to relevant issue(s)/PR(s).
+
+## Prefer to Email?
+
+Email the person listed as maintainer in the `DESCRIPTION` file of this repo.
+
+Though note that private discussions over email don't help others - of course
+email is totally warranted if it's a sensitive problem of any kind.
+
+## Thanks for contributing!
+
+This contributing guide is adapted from the tidyverse contributing guide
+available at
+<https://raw.githubusercontent.com/r-lib/usethis/master/inst/templates/tidy-contributing.md>