new testing instructions

Author: cpsievert

CONTRIBUTING.md

@@ -6,29 +6,16 @@ See the [opening issues template](https://github.com/ropensci/plotly/blob/master
 
 ## Development guidelines
 
-If you'd like to contribute changes to plotly, we use [the GitHub flow](https://guides.github.com/introduction/flow/index.html) for proposing, submitting, reviewing, and accepting changes. If you haven't done this before, Hadley Wickham provides a nice overview of git (<http://r-pkgs.had.co.nz/git.html>), as well as best practices for submitting pull requests (<http://r-pkgs.had.co.nz/git.html#pr-make>). We also recommend using his style guide when writing code (<http://adv-r.had.co.nz/Style.html>).
+If you'd like to contribute changes to plotly, we use [the GitHub flow](https://guides.github.com/introduction/flow/index.html) for proposing, submitting, reviewing, and accepting changes. If you aren't familiar with git and/or GitHub, we recommend studying these excellent free resources by [Hadley Wickham](http://r-pkgs.had.co.nz/git.html) and [Jenny Bryan](http://happygitwithr.com). We also prefer R coding style that adheres to <http://style.tidyverse.org/>.
 
-If your pull request fixes a bug, or implements a new feature, it's a good idea to write a test (<http://r-pkgs.had.co.nz/tests.html>) to demonstrate it's working. If you'd like to closely simulate the tests that run when you submit your pull request, open R under your local plotly git repo, then do the following:
+If your pull request fixes a bug, or implements a new feature, please [write a test via testthat](http://r-pkgs.had.co.nz/tests.html) to demonstrate it's working. The testing suite can optionally leverage **vdiffr**'s `expect_doppelganger()` to monitor changes via svg -- to make sure those those tests run, do:
 
 ```r
-# the pull request number is arbitrary when running locally
-Sys.setenv('TRAVIS_PULL_REQUEST' = '1')
-Sys.setenv('TRAVIS_COMMIT' = substr(system('git rev-parse HEAD', intern = T), 1, 7))
-devtools::load_all(); source('tests/testthat.R', chdir = TRUE)
+Sys.setenv("USE_VDIFFR" = "true")
+devtools::test()
 ```
 
-You can also build a ggplot2/plotly comparison table. To do so, you'll first need to clone the [plotly-test-table](https://github.com/cpsievert/plotly-test-table) repo.
-
-```shell
-$ git clone https://github.com/cpsievert/plotly-test-table.git ../plotly-test-table
-```
-
-Then, from R:
-
-```r
-Sys.setenv('PLOTLY_TABLE' = 'TRUE')
-devtools::load_all(); source('tests/testthat.R', chdir = TRUE)
-```
+Say your changes produce changes in the corresponding svg of existing tests and/or you want to do write new tests (hooray!). In that case, you'll want to run `vdiffr::manage_cases()`, which should eventually launch a shiny app of "mismatched" and "new" to visually inspect before approving the changes.
 
 ## Code of Conduct