Improved documentation for gitbookConfigURL

README.md

@@ -54,6 +54,28 @@ Put this in your book.json:
 A `<select>` element will be created with the given `options` and placed at the top of the book summary. When the user selects one of the options, they are taken to that URL.
 
 The `gitbookConfigURL` variable is a publicly accessible URL to your `book.json`. If this is present, the plugin will fetch the latest config when the page loads, so even older versions of your book will have updated `options`.
+However, each version of your book needs to have book.json, which defines the `gitbookConfigURL` variable. It should also define at least an option for the current version, with `selected` set to true. 
+The list of options will be overwritten by the config in `gitbookConfigURL`, but the current selected version will be matched according the value (URL) of the previous selected option.
+
+#### An example how to define 2 versions with a common config
+
+Let's have 2 versions in branches v1 and v2. We want to define a common config in another branch/repository, so that we need to update only single config file if we add new versions later.
+
+1. Create a completely new branch (can be empty) or a new empty repository
+2. Create a `book.json` file in the new config branch/repo with configuration for `versions` plugin in `pluginsConfig`. Define options with value and text for v1 and v2.
+3. Create a `book.json` file in v1 and v2 branches as in the above example. Add `gitbookConfigURL` pointing to raw book.json file in your config branch
+4. For both v1 and v2 branches, add single element into `options` in book.json - the`value` should point to the gitbooks URL matching the v1 or v2 branch, the `text` is the label s defined in the conig branch, and `selected` should be set to true
+
+You will end up with 3 `book.json` files in 3 branches. Branch v1 contains a single option pointing to the v1 documentation, v2 contains again a single option pointing to the v2 documentation. Both also contain referece to the common book.json config in another branch. 
+The book.json file in the config branch is the only file you need to modify from now on. 
+
+Let's add a new version:
+
+1. Create v3 branch (based on v2 branch)
+2. Modify `book.json` acordingly - update the value with correct v3 URL and text (label)
+3. Modify `book.json` in config branch - copy the option from v3 branch (after modifications) and add it to the list of options
+
+And you are done. All 3 options will be available in all 3 versions of the documentation.
 
 ### Credits