Add vignettes to demonstrate usage of the package

[ha0ye]

In prep for a submission to CRAN and JOSS, it would be ideal to have several vignettes with short examples of using the portalr functions to get the Portal data and do some simple analyses or plots.

Here's a suggested workflow for writing a vignette:

1. Make a new Rmarkdown file within the vignettes folder.

• If you're using RStudio, you can select "From Template" -> "Package Vignette (HTML)"
• You can also try devtools::use_vignette("<vignette name>")

2. Follow standard Rmarkdown for writing the vignette.
3. Use devtools::build_vignettes() to build and include the vignette files to the package.
4. (standard Git add, commit, push, pull-request)

Note, if you use packages in the vignette that aren't dependencies of portalr, make sure to include them in the DESCRIPTION file under Suggests:

There's an example already in the vignettes folder that you can take a look at.

[gmyenni]

• Estimating missing data/Adjusting for effort

• Energy

• Phenology