Documentation about bases and relative paths

[philomory]

When you have a "base" file that is in a different directory from your main file, like this

bases:
  - ./foo/example.yaml

relative paths in ./foo/example.yaml are interpreted as relative to the main ./helmfile.yaml, rather than relative to the base file.

This makes sense in light of how base files are interpreted (they're more-or-less just read and virtually appended to the main file), but it's probably worth explicitly documenting that it works this way in the documentation about relative paths, especially since this is different from how relative paths work in sub-helmfiles included via the helmfiles key.