doc(README): add a migrating section

Author: goldenstein64

README.md

@@ -43,14 +43,11 @@ Addon developers should use an addon file structure like this:
 .
 ├── [addon name]-[version]-[revision].rockspec  # required
 ├── library/     # optional
-├── plugin.lua   # optional
-└── config.json  # deprecated
+└── plugin.lua   # optional
 ```
 
 See the [Creating an Addon](https://luals.github.io/wiki/addons/#creating-an-addon) section for more information on the individual components.
 
-The `config.json` file is considered deprecated when used with this installer. The `words` and `files` fields will not be read, and the `settings` field is superceded by the rockspec's `build.settings` field.
-
 A `.rockspec` file should be included with the following block:
 
 ```lua
@@ -110,6 +107,10 @@ build = {
 }
 ```
 
+### Migrating
+
+A `config.json` file can be included with the addon, but it is considered deprecated when used with this installer. The `words` and `files` fields will not be read, and the `settings` field is superceded by the rockspec's `build.settings` field.
+
 ## Building
 
 ```sh