Put pandoc on the path during a check

[jimhester]

At least on windows pandoc (as installed by RStudio) is unlikely to be on the system PATH, so R CMD check can't find it, and generates a note.

N checking top-level files
Files 'README.md' or 'NEWS.md' cannot be checked without 'pandoc' being installed.

However rmarkdown::pandoc_available() is TRUE, so we could probably put that the pandoc on the PATH, e.g. using withr::with_path(dirname(rmarkdown::pandoc_exec())) before running R CMD check.

[kmaheshkulkarni]

No Need to Update pandoc Simple way to resolve this issue Just Add
README.md in .Rbuildignore File

[gaborcsardi]

Closed by #132