docs: add slightly tweaked boilerplate CONTRIBUTING.md (#1423)

Author: aviator-app[bot]

.github/CONTRIBUTING.md

@@ -0,0 +1,56 @@
+# Contributing to igraph
+
+This outlines how to propose a change to igraph.
+
+## Fixing typos
+
+You can fix typos, spelling mistakes, or grammatical errors in the documentation directly using the GitHub web interface, 
+as long as the changes are made in the _source_ file. 
+This generally means you'll need to edit [roxygen2 comments](https://roxygen2.r-lib.org/articles/roxygen2.html) in an `.R`, not a `.Rd` file. 
+You can find the `.R` file that generates the `.Rd` by reading the comment in the first line.
+
+## Bigger changes
+
+If you want to make a bigger change, it's a good idea to first file an issue and make sure someone from the team agrees that it’s needed. 
+If you’ve found a bug, 
+please file an issue that illustrates the bug with a minimal [reprex](https://www.tidyverse.org/help/#reprex) 
+(this will also help you write a unit test, if needed).
+See the tidyverse guide on [how to create a great issue](https://code-review.tidyverse.org/issues/) for more advice.
+
+### Pull request process
+
+*   Fork the package and clone onto your computer. 
+    If you haven't done this before, you can use `usethis::create_from_github("igraph/rigraph", fork = TRUE)`.
+
+*   Install all development dependencies with `pak::pak()`, 
+    and then make sure the package passes R CMD check by running `devtools::check()`. 
+    If R CMD check doesn't pass cleanly, it's a good idea to ask for help before continuing. 
+*   Create a Git branch for your pull request (PR). 
+    You can use [usethis](https://usethis.r-lib.org/articles/pr-functions.html), GitHub Desktop, etc.
+
+*   Make your changes, commit to git, and then create a PR.
+    The title of your PR should briefly describe the change.
+    The body of your PR should contain `Fixes #issue-number`.
+
+### Code style
+
+*   New code should follow the tidyverse [style guide](https://style.tidyverse.org). 
+    You can 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), 
+   with [Markdown syntax](https://cran.r-project.org/web/packages/roxygen2/vignettes/rd-formatting.html), 
+   for documentation.  
+
+*  We use [testthat](https://cran.r-project.org/package=testthat) for unit tests. 
+   Please add test cases for the change you are proposing, or ask us for help.
+
+### Test files
+
+We strive to align script and test names. 
+This is still a work-in-progress but the goal is to have code from `R/name.R` tested in `tests/testthat/test-name.R`.
+This allows easy toggling between the two files thanks to `usethis::use_test()` and `usethis::use_r()`.
+
+## Code of Conduct
+
+Please note that the igraph project is released with a [Contributor Code of Conduct](https://igraph.org/code-of-conduct.html). 
+By contributing to this project you agree to abide by its terms.