Update installation section

Adds make help

"disable_script" function breaks "grub-mkconfig"

* "disable_script" function breaks "grub-mkconfig":
  * Replace "return 1" by "exit 0".

Fix bug Create entries with matching version (Antynea#170)

* Fix Antynea#169 
  * Create entries with matching version doesn't work properly:
  Adds missing "else continue;"

Adds missing newline

* Adds missing newline:
  "\n" was missing.

Makefile: Make sure make is invoked with appropriate permissions (Ant…

…ynea#146)

[make help] mention `INITCPIO` switch (Antynea#137)

This commit is an expansion to :
Antynea@87b8163

Since a new `make` parameter has been introduced, it should be mentioned
in the output of the `help` target.

Also, the table is being expanded by a new column describing the type of
data that each parameter may be assigned, like paths or boolean values.

Adds a new config variable "GRUB_BTRFS_ROOTFLAGS" (Antynea#133)

Script:

Improves the rootflags option in the kernel command line:
- Remove "rw" flag.
- Parse fstab mountoptions and add them to rootflags:
Adds functionality to read in  snapshot specific mount flags
from the respective fstab and uses them in combination with the new 
config option, while the latter is prioritized.
Both places can, but don't need to define mount flags. If either of them
doesn't, it will simply not be taken into account.

Config file:

Adds a new config variable "GRUB_BTRFS_ROOTFLAGS":
- The new variable is used to include user-defined rootflags in the 
kernel command line of generated snapshot bootmenu entries.
See config file for more infos.

Corrects an erroneous variable (Antynea#128)

Fix Antynea#127
Replaces "GRUB_BTRFS_DIRNAME" variable to match with new variable "GRUB_BTRFS_GRUB_DIRNAME" in config file.

Corrects a mistake

Corrects the duplicate that appeared
Line16 should be :
`@install -Dm644 "initramfs/Arch Linux/overlay_snap_ro-hook" "$(LIB_DIR)/initcpio/hooks/grub-btrfs-overlayfs"		# Arch Linux only`

Fix indentation

My IDE was set up incorrectly.
Convert all tab indentation to space.