Merge branch '5-write-readme-file' into 'master'

Resolve "Write readme file"

Closes #5

See merge request !3

README.md

+CryptOps-enabled Initrd
+=======================
+
+This repository contains files that need to be added or changed in a "vanilla"
+initrd in order to be able to use the
+[CryptOps](https://code.greenhost.net/open/cryptops) toolkit to set up an
+encrypted VPS. CryptOps is included as a subrepository, to be able to track the
+changes to the API with the changes of this initrd. Use the `build.sh` script to
+copy the CryptOps api and client binaries to the correct location in the initrd
+before building it.
+
+More information can be found on the [documentation
+website](https://cryptops.com)
+
+## VPS setup
+
+Some variables need to be passed to the initrd with kernel options. They are
+retrieved from `/proc/cmdline`. [Here's the official
+documentation.](https://github.com/torvalds/linux/blob/master/Documentation/filesystems/nfs/nfsroot.txt)
+
+- `root=/dev/mapper/xvda1_crypt`: The partition that needs to be mounted as root
+- `cryptroot=/dev/xvda`: The device that needs to be partitioned and encrypted
+- `ip=213.108.110.71::213.108.110.126:255.255.255.128:encrypted-system:eth0:off`
+  IP information:
+  `ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:<dns0-ip>:<dns1-ip>`
+
+### Startup Notification
+
+Some extra variables are used in Greenhost's case to be able to use the Cosmos
+Service Centre API to send a message to the owner of a VPS after it has been
+restarted. These variables are used in /scripts/local-top/cryptroot-api to make
+`$notification_command`.
+
+If you don't run on Greenhost infrastructure, change
+that variable to something else to notify you of a (re)boot. 
+
+If you do not have a notification_command, CryptOps will work fine, but when
+your VPS reboots, you run the risk of not knowing this and your VPS will stay in
+the initrd, waiting for you to unlock the disk.
+
+- `api_key=<key>`: The *Bearer* authentication key for using the Cosmos API
+- `instance_id=###`: The unique identifier of the VPS
+- `cosmos_url=https://management.greenhost.nl`: The URL to the Cosmos API
+
+A VPS is assumed to have one drive that needs to be encrypted (additional drives
+can be encrypted manually from the booted operating system). The drive will be
+partitioned into an encrypted root partition (in this example `xvda1_crypt`) and
+a not encrypted data partition. The latter is used to save SSH keys, so the user
+can log into the SSH shell running in the initrd.
+
+## File structure
+
+The folder `crypt-initrd-extra-files` contains all the files that need to be
+inserted in the root directory of an initrd to make CryptOps work. Some things
+stand out:
+
+- The folder contains some symlinks pointing to a nonexistent directory
+  `conf/persistent/` (for example the `home_root` file). The persistent file
+  system is mounted in this directory by
+  `scripts/init-premount/mount_persistent_config`