CRAN Submission Review

[apulsipher]

This pull request makes updates to the package to prepare it for CRAN submission:

• Adds the ForeSITE badge to the README file, which points back to the EpiForeSITE GitHub organization
• Updates automated R CMD check to cover multiple OS and to build the final submission artifact
• Resolves NOTEs in R CMD check by
  • Adds a DCF LICENSE file alongside the existing LICENSE.md
  • Adds paper/ dir to .Rbuildignore
  • Adds package anchors to two link targets in create_stars_raster.R and polygon_to_raster.R
• Adds cran-comments.md file
• Adds CRAN installation instructions to README in anticipation of package acceptance