close #125: add a contributing guide

.github/PULL_REQUEST_TEMPLATE.md

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+To contribute a new article template to this package, please make sure you have done the following things (note that `journalname_article` below is only an example name):
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+- [ ] Unless you have done it in any other RStudio's projects before, please sign the [individual](https://rstudioblog.files.wordpress.com/2017/05/rstudio_individual_contributor_agreement.pdf) or [corporate](https://rstudioblog.files.wordpress.com/2017/05/rstudio_corporate_contributor_agreement.pdf) contributor agreement for a significant pull request (it is fine not to sign it if a PR is only intended to fix a few typos). You can send the signed copy to <jj@rstudio.com>.
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+- [ ] Add the `journalname_article()` function to `R/journalname_article.R`. If the article needs CSL to process citations, see the function `acm_article()` for an example; otherwise see `asa_article()` for an example. If you don't know what CSL means, see the latter.
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+- [ ] Add the Pandoc LaTeX template `inst/rmarkdown/templates/journalname_article/resources/template.tex`.
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+- [ ] Add a skeleton article `inst/rmarkdown/templates/journalname_article/skeleton/skeleton.Rmd`.
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+- [ ] Add a description of the template `inst/rmarkdown/templates/journalname_article/template.yaml`.
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+- [ ] Please include the document class file (`*.cls`) if needed, but please do not include standard LaTeX packages (`*.sty`) that can be downloaded from CTAN.
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+- [ ] Update Rd and namespace (could be done by `devtools::document()`).
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+- [ ] Update NEWS.
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+- [ ] Update README with a link to the newly supported journal.
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+- [ ] Add a test to `tests/testit/test-formats.R`.
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+- [ ] Add your name to the list of authors `Authors@R` in DESCRIPTION. You don't need to bump the package version in DESCRIPTION.
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+Thank you!