Add missing arch_instructions.md

arch_instructions.md

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+# Install hashboot on Arch Linux
+## Clone repository
+First, clone the git repository:
+
+`git clone https://github.com/tastytea/hashboot.git`
+
+Then distribute the files to the proper path on your drive.
+
+- the hashboot script should go to /usr/bin
+- everything in /init/systemd should go to /etc/systemd/system
+
+Then make the hashboot script executable with the command:
+
+`sudo chmod a+x /usr/bin/hashboot`
+
+Then run this command once to generate the configuration file and initial files:
+
+`sudo hashboot index`
+
+## Setting up systemd
+Run the command:
+
+`systemctl enable hashboot.service`
+
+## Setting up the post-install kernel hook
+Pacman uses [alpm hooks](https://www.archlinux.org/pacman/alpm-hooks.5.html) to facilitate triggering an operation after a file or package has been modified by pacman. The system hooks are stored in the `/usr/share/libalpm/hooks` directory.
+
+Since `hashboot index` should be run after every kernel update (and after all other post-install actions have completed), create the file `zzz-hashboot.hook` in `/usr/share/libalpm/hooks` and give it the following contents:
+
+```
+[Trigger]
+Operation = Install
+Operation = Upgrade
+Type = Package
+Target = linux
+
+[Action]
+Description = Generating hashboot checksums of MBR and /boot...
+When = PostTransaction
+Exec = /usr/bin/hashboot index
+```
+
+Now when you update the kernel, hashboot will generate new files.
+
+## Making sure hashboot runs at boot
+Immediately after booting, you can view the status of the hashboot service with the command
+
+`systemctl status hashboot.service`
+
+It should print something like:
+
+```
+* hashboot.service - Check for changes made to the boot partition since shutting down
+   Loaded: loaded (/etc/systemd/system/hashboot.service; enabled; vendor preset: disabled)
+   Active: inactive (dead) since Sun 2019-05-12 09:27:48 PDT; 20s ago
+  Process: 1292 ExecStart=/sbin/hashboot check (code=exited, status=0/SUCCESS)
+ Main PID: 1292 (code=exited, status=0/SUCCESS)>
+ 
+May 12 09:27:47 hostname systemd[1]: Starting Check for changes made to the boot partition since shutting down...
+May 12 09:27:48 hostname systemd[1]: hashboot.service: Succeeded.
+May 12 09:27:48 hostname systemd[1]: Started Check for changes made to the boot partition since shutting down.
+```