Update the PR template about CLA assistant

Author: cderv

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,7 @@
 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):
 
-- [ ] 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>.
+- [ ] This project uses a Contributor Licence Agreement (CLA) that you'll be asked to sign when opening a PR. This is required for a significant pull request (it is fine not to sign it if a PR is only intended to fix a few typos). We use a tool called CLA assistant for that.  
+You could also, unless you have done it in any other RStudio's projects before, 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. You can send the signed copy to <jj@rstudio.com>.
 
 - [ ] Add the `journalname_article()` function to `R/article.R` if the output format is simple enough, otherwise create a separate `R/journalname_article.R`.