add more non-standard files to .Rbuildignore
add more non-standard files to .Rbuildignore

eliminates this note from R CMD check:

* checking top-level files ... NOTE
Non-standard files/directories found at top level:
  ‘man-roxygen’ ‘todo.org’

aronlindberg/rgithub,cscheid/rgithub,akhmed1/rgithub,s-u/rgithub