R CMD check warnings (--as-cran)

[jakubnowicki]

Additional checks from --as-cran option produce a warning:

* checking top-level files ... WARNING
Conversion of ‘README.md’ failed:
[WARNING] This document format requires a nonempty <title> element.
  Please specify either ‘title’ or ‘pagetitle’ in the metadata,
  e.g. by using --metadata pagetitle="..." on the command line.
  Falling back to ‘README’
File docs/style.css not found in resource path
Non-standard file/directory found at top level:
  ‘depends.Rds’

[dokato]

Readme is not mandatory for CRAN submission so we can simply add it to .Rbuildignore