adding contribution instructions

.github/CONTRIBUTING.md

@@ -0,0 +1,47 @@
+# Contributing to serocalculator
+
+This outlines how to propose a change to serocalculator.
+For a detailed discussion on contributing to this and other tidyverse packages, please see the [development contributing guide](https://rstd.io/tidy-contrib) and our [code review principles](https://code-review.tidyverse.org/).
+
+## 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 our 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, we recommend using `usethis::create_from_github("UCD-SERG/serocalculator", fork = TRUE)`.
+
+*   Install all development dependencies with `devtools::install_dev_deps()`, 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). We recommend using `usethis::pr_init("brief-description-of-change")`.
+
+*   Make your changes, commit to git, and then create a PR by running `usethis::pr_push()`, and following the prompts in your browser.
+    The title of your PR should briefly describe the change.
+    The body of your PR should contain `Fixes #issue-number`.
+
+*  For user-facing changes, add a bullet to the top of `NEWS.md` (i.e. just below the first header). Follow the style described in <https://style.tidyverse.org/news.html>.
+
+### 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. 
+   Contributions with test cases included are easier to accept.  
+
+## Code of Conduct
+
+Please note that the serocalculator project is released with a
+[Contributor Code of Conduct](CODE_OF_CONDUCT.md). By contributing to this
+project you agree to abide by its terms.