Suggest use_build_ignore() instead of add_build_ignore.  Also minor copy editing.  Addresses issue hadley/devtools#666
Check whether cran-comments.md exists before asking many other questions.  Closes hadley/devtools#666
Merge pull request #667 from az0/master

Be more helpful concerning cran-comments.md
Comma missing R/release.r line 225
Merge pull request #669 from madwsa/master

Comma missing R/release.r line 225