Clarify docs around pagination options

[jimniels]

I was having a hard time figuring out how this particular plugin worked with the configuration options in the official hexo docs. After looking at the code, it appears it inherits those values (if present), so this was an attempt to merely clarify that in the README.

For example, if you have per_page in your hexo config, then this plugin is going to use that, i.e.

# _confg.yaml
# Both of these accomplish the same thing
per_page: 40       # specified in hexo docs
index_generator: # specified in hexo-generator-index docs
  per_page: 40

[coveralls]

Coverage remained the same at 100.0% when pulling 5b4889e on jimniels:patch-1 into c7e2eab on hexojs:master.