added link to example images.json file

The build from source instructions need users to download the images.json file. This commit adds the link to download the example file used for new PiBakery installations.

Author: davidferguson

README.md

@@ -36,7 +36,7 @@ If you're on macOS it's important you run `npm run setup` the first time [to cre
 
 Then you'll need to download and extract the Raspberry Pi operating system (currently only raspbian and raspbian lite) .img files into the `os/` directory. If you're on Windows or Linux, the directory is inside the `pibakery` folder, and if you're on macOS, it's located `/Library/Application Support/PiBakery/os`. The operating systems can be downloaded in `.7z` format for the [pibakery-raspbian releases page](https://github.com/davidferguson/pibakery-raspbian/releases).
 
-Lastly you'll need to create a file `images.json` inside the `os` folder which tells PiBakery which operating systems you have installed and available for use. Download the example file from [not-created-yet](), save it as `images.json` into the `os` folder where you stored the .img files, and then change the values of the `installed` paramater for each of the operating systems depending on which ones you have installed.
+Lastly you'll need to create a file `images.json` inside the `os` folder which tells PiBakery which operating systems you have installed and available for use. Download the example file from [the pibakery-raspbian repo](https://raw.githubusercontent.com/davidferguson/pibakery-raspbian/master/images.json), save it as `images.json` into the `os` folder where you stored the .img files, and then change the values of the `installed` paramater for each of the operating systems depending on which ones you have installed.
 
 You can then run PiBakery using
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