Ctags is always needed by tagbar, even when only editing Go files

[Gys]

See my issue here:
preservim/tagbar#242

I think its a good idea to mention in the readme that an up to date ctags is also always needed.

[jstemmer]

Thanks for the info, however this is a Tagbar issue. Their excellent readme, to which I already link, mentions that exuberant-ctags 5.5 is required.