Created Prerequisites (markdown)

Prerequisites.md

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+### Generate an application key
+
+Before starting Invoice Ninja via Docker make sure you generate a valid application key. If you are not sure what an application key is, please visit [this blog post](https://tighten.co/blog/app-key-and-you/).  
+
+To generate an application just run
+
+```shell
+docker run --rm -it invoiceninja/invoiceninja php artisan key:generate --show
+```
+
+This will generate an application key for you which you need later.
+
+### Create folders for data persistence
+
+To make your data persistent, you have to mount `public` and `storage` from your host to your containers.
+
+1. Create two folder on your host, e. g. `/var/invoiceninja/public` and `/var/invoiceninja/storage`
+2. Mount these folders into your container - see [usage](#usage)
+
+You can create these folders wherever you want on your host system.
+
+:warning: When using host mounted folder for persistence, make sure they are owned by the proper user and group. As we run Invoice Ninja without `root` , we use a separate user, the folders on the host system need to be owned by uid `1000` and a gid `101`.  
+
+Run this on your host system
+
+```shell
+chown -R 1000:101 /var/invoiceninja/public /var/invoiceninja/storage
+```
+
+to apply the proper permission to the folders. This also applies to the `docker-compose` setup when using [bind-mounted host directories](https://github.com/invoiceninja/dockerfiles/blob/master/docker-compose.yml#L17).
+
+
+### PhantomJS key
+
+The PhantomJS key is set to `a-demo-key-with-low-quota-per-ip-address`. This demo key is limited to 100 requests per day.
+
+To set a different key feel free to add `-e PHANTOMJS_CLOUD_KEY='<INSERT YOUR PHANTOMJS KEY HERE>'` to thee docker command below.
+
+For further configuration and toubleshotting regarding PhantomJS and Invoice Ninja [see documentation here](https://docs.invoiceninja.com/configure.html?#phantomjs).