added description for local builds and variables

README.md

@@ -15,13 +15,54 @@ You'll need the following dependencies:
 * virtualbox (for virtualbox support)
 * VMware Workstation Pro (for vmware support)
 
-## How to start the build process
+## variables
+Here is an overview over all variables you can set in `vagrant.json` or
+`local.json`:
+
+* `iso_url`: the url to the ISO. This can be an url or a filepath
+  beginning with `file://`
+* `iso_checksum_url`: the url to the checksum file. This can be an url
+  or a filepath beginning with `file://`
+* `iso_checksum_type`: this specifies the hashing algorithm for the
+  checksum.
+* `disk_size`: this specifices the disk size in bytes.
+* `memory`: this specifies the size of the RAM in bytes.
+* `cpus`: this specifies the number of cores for your VM.
+* `headless`: this sets GUI on or off.
+* `atlas_token`: here you can specify the atlas token for uploading your
+  box to the vagrantcloud. If you don't have any atlas token you can
+  ignore this variable. But then don't be suprised when the process
+  fails. The boxes are build, they just haven't been uploaded.
+* `write_zeroes`: this variable is empty. if you set any string in this
+  variable it will fill the box with zeros to reduce the size. **DO NOT
+  use this if you are running a SSD. It will harm your SSDs lifetime**
+* `boot_wait`: this specifies the time packer should wait for booting up
+  the ISO before entering any command.
+
+## how to start the build process locally
+Edit the `local.json` before you start the build. set the right
+`iso_url` and the right `iso_checksum_url`. Then you can start the build
+for virtualbox only with the following command:
+
+On Arch Linux:
+
+`packer-io build -only=virtualbox-iso -var-file=local.json vagrant.json`
+
+On any other system:
+
+`packer build -only=virtualbox-iso -var-file=local.json vagrant.json`
+
+**Note** this is because of the name conflict with the AUR-Helpertool
+`packer` on Arch Linux.
+
+## how to start the build process for official builds
+The official builds are done on our Arch Linux Buildserver.
 
 On Arch Linux:
 
 `packer-io build vagrant.json`
 
-On any other System:
+On any other system:
 
 `packer build vagrant.json`