Document which dir to use for cache

[oalders]

It looks to me that if I want to cache installed modules I need to add:

cache:
  directories:
    - ~/perl5

I couldn't find this in the docs. If it's worth adding, I'm happy to create a PR.

[autarch]

That is indeed the correct directory to cache. See my blog post for some more info - https://blog.urth.org/2017/10/01/caching-and-the-travis-perl-helpers/

[oalders]

@autarch thanks. So, should this be documented?

[autarch]

Yes, I think so.