Add docs/ to .gitignore

[rudeboybert]

@ismayc Is there any reason to keep docs/ synced on GitHub, since travis-ci renders the book to the gh-pages branch? I feel like we should only sync changes to .Rmd and .yml files and images/data files, otherwise it increases the chances of merge conflicts and makes for harder commit/branch comparisons.

[rudeboybert]

Also perhaps instead have travis-ci copy the contents of the gh-pages rendered book to the master branch docs/ folder in the case there are still floating GitHub pages https://moderndive.github.io/moderndive_book/ links out there?

Updated: nevermind, this will only cause merge conflicts with our local versions of docs/

[ismayc]

I like being able to have a backup just in case travis isn’t working. Merge conflicts should be pretty minimal and if they do exist we can just pull down and then recompile. The docs folder doesn’t have to be updated that frequently but can still be useful at times I think.

[rudeboybert]

fine by me