Adding README clarification for pathPrefix arg. …

[EdPike365]

… It was marked required but did not specify that it could not be empty string. I do not need the ability to leave it empty. But the plugin ate the error, only "createPage" threw one (below).

To Reproduce

I was using paginate in gatsby-node.js In the Config, I left the pathPrefix blank, eg: pathPrefix: "", I ran gatsby develop and got an Error 11323 from createPage:

Your site's "gatsby-node.js" must set the page path when creating a page.

The page object passed to createPage:

{
"path": "",
"component": "C:\\Users\\EdPike365\\code\\gatsby-rich-text\\src\\templates\\blog-posts.js",
"context": {
"basePath": "",
"paginationPath": "",
"pageNumber": 0,
"humanPageNumber": 1,
"skip": 0,
"limit": 7,
"numberOfPages": 1,
"previousPagePath": "",
"nextPagePath": ""
}
}

Expected Behaviour

Plugin code should throw an error when it encounters an empty required string.

Additional Context

I tried to debug the code where you create a "wrapped" config object, but the code seemed to magically flake. I barely know node and I don't know Lodash at all so that is probably the problem on my end. I come from planet Java, JSP City. I'll get better... This is my first PR anywhere, ever! Thanks for making the plugin.

[chmac]

Thanks for pointing this out. I've updated the readme to make it clearer, but slightly differently to your proposal.