(grub) added entry on grub

grub.md

@@ -0,0 +1,25 @@
+# grub
+
+This file contains collected knowledge on `grub`.
+
+
+## Table of contents
+
+<!--toc:start-->
+- [grub](#grub)
+  - [Table of contents](#table-of-contents)
+<!--toc:end-->
+
+
+## reinstalling grub
+
+use the following command to (re)install grub:
+
+```sh
+sudo grub-install --efi-directory=<boot-dir> --target=x86_64-efi <boot-partition>
+# example
+sudo grub-install --efi-directory=/boot --target=x86_64-efi /dev/nvmen1p1
+```
+
+**boot-dir** is the mount point of the bootloader partition.
+**boot-partition** is the device path to the partition containing the bootloader