Note in R CMD check - Non-standard file

Note in R CMD check - Non-standard file

i am creating and checking a package. I have put the Rnw file and images
it uses in the vignette directory and the pdfof the Rnw file in inst/doc
directory.
Then i run R CMD check pkgnameand R CMD build pkgname - both of them run
without any notes or warnings or errors. However when i run R CMD check
--as-cran-pkgname_version.tar.gz, i get the following note:
checking top-level files ... NOTE
Non-standard file found at top level: 'build'
i see that there is a directory called 'build' inside the tarball.
what is going wrong? what should i do?