Update .gitignore to add tags

`/doc/tags` gets auto-created upon plugin install (i am using Vundle plugin manager, believe this is vim behaviour). When using git submodules, the existence of `doc/tags` causes git errors indicating untracked content.
Many vim plugins account for this, and add `tags` or `doc/tags` to their .gitignore files -
for example: [vim-fugitive](https://github.com/tpope/vim-fugitive/blob/master/.gitignore), [nerdcommenter](https://github.com/scrooloose/nerdcommenter/blob/master/.gitignore) or [tabular](https://github.com/godlygeek/tabular/blob/master/.gitignore).
Have about 60 plugins, and only 4 currently missing `tags` in their .gitignore - please consider accepting this PR.

.gitignore

@@ -0,0 +1,2 @@
+/doc/tags
+tags