adding .Rbuildignore file

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/phylobase@893 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
tagging the 0.6.8 CRAN release.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/phylobase@894 edb9625f-4e0d-4859-8d74-9fd3b1da38cb