clarify documentation on how to use captions in cover images; related…

… to daattali#740

README.md

@@ -137,7 +137,7 @@ Parameter   | Description
 title       | Page or blog post title
 subtitle    | Short description of page or blog post that goes under the title
 tags        | List of tags to categorize the post. Separate the tags with commas and place them inside square brackets. Example: `[personal, analysis, finance]`
-cover-img   | Include a large full-width image at the top of the page. You can either provide the path to a single image (eg. `"/path/to/img"`) , or a list of images to cycle through (eg. `["/path/img1", "/path/img2"]`). If you want to add a caption to an image, then the image should be provided as `{"/path/to/img" : "Caption of image"}`.
+cover-img   | Include a large full-width image at the top of the page. You can either provide the path to a single image (eg. `"/path/to/img"`) , or a list of images to cycle through (eg. `["/path/img1", "/path/img2"]`). If you want to add a caption to an image, then you must use the list notation (use `[]` even if you have only one image), and each image should be provided as `"/path/to/img" : "Caption of image"`.
 thumbnail-img | For blog posts, if you want to add a thumbnail that will show up in the feed, use `thumbnail-img: /path/to/image`. If no thumbnail is provided, then `cover-img` will be used as the thumbnail. You can use `thumbnail-img: ""` to disable a thumbnail.
 comments    | If you want do add comments to a specific page, use `comments: true`. Comments only work if you enable one of the comments providers (Facebook, disqus, staticman, utterances) in `_config.yml` file. Comments are automatically enabled on blog posts but not on other pages; to turn comments off for a specific post, use `comments: false`.