Create CONTRIBUTING.md

Author: SaicharanKandukuri

CONTRIBUTING.md

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+## INTRODUCTION
+fs-cooks uses debootstrap to pull core Linux packages to build linux traballs, all implementations are developer-friendly and mostly kinda [udroid](https://github.com/RandomCoderOrg/ubuntu-on-android) centric (for now)
+
+### filestructure
+<hr>
+
+#### Top-Level (1)
+the top-level contains some example build scripts which are lightweight to build and beginning of directories
+```
+.
+├── build-hirsute-raw.sh -> cook.sh
+├── build-impish-raw.sh
+├── cook.sh
+├── core
+├── out
+├── plugins
+├── README.md
+└── setup.sh
+```
+here:
+- **core**: contains binaries and different sources that used in the build process
+- **out**: All the filesystem and tarball packages are created here
+- **plugins**: contains scripts that combine core sources to make tarball building easy
+<hr>
+
+**important files/folders to notice**
+###### `~/core/defaults`
+defaults folder contains some heavy templates to build DE ready tarballs like mate,xfce4,kde
+###### `~/plugins/envsetup`
+contains functions that can be used for cmd line building and integrating in scripts
+- useful functions in `envsetup`:
+
+<kbd>do_mount()</kbd>: mounts the rootfs directories in recursive after checking is directories already mounted
+> takes one argument: location of rootfs directorie
+
+```bash
+do_mount "/path/to/fs"
+```
+
+<kbd>do_debootstrap()</kbd> bootstraps linux to a directories of specified arch
+```bash
+do_debootstrap "out/udroid-test" "arm64"
+```
+###### TODO ( need to write more )
+
+# AIM
+> in one line: able to produce multiarch ubuntu hirsute & impish tarballs with
+
+```bash
+source plugins/envsetup
+SUITE=impish
+do_debootstrap "out/fs" "arm64"
+```
+### Quick build scripts
+- `build-impish-raw.sh`: to build raw ubuntu 21.10 tarballs
+- `build-hirsute-raw.sh`: to build raw ubuntu 21.04 tarballs
+> others are experimental ( may break things )
+
+### functions
+> recommended for devolopers
+ 
+- `do_mount()`: mounts target filesystem directories ( recursive mode ) `do mount /path/to/fs`
+- `is_mounted()`: checks is filesystem mounted to host
+- `list_parser()`: convets new line separated contents in a file to list variable
+- `depends_on()`: for locking dependencies ( checks is a package is installed  with `command`)
+- `dpkg_depends_on()`: check for dependencies with dpkg for non-binarie bundle applications
+- `see_for_directory()`: check for directories & set `es` to false if not found
+- `no_to_directory()`: exits if directory given is present
+- `foreign_arch()`: checks does target arch matches with host architecture
+- `includes_packages()`: takes care of extrapackges when a variable `INCLUDE_PACKAGES` is set with packages
+- `do_debootstrap()`: bootstraps linux to with target arch to target directorie
+- `do_second_stage()`: if foreign arch triggers second stage
+- - `do_qemu_user_emulation()` sets up qemu binaries in chroot
+- `do_chroot_ae()`: to run command in chroot
+- - `run_cmd()`: alternative for `do_chroot_are()`
+- `do_compress()`:  takes care of compressing tarballs without messy device file
+- - `do_tar_gzip()`: to compress in gzip format
+- - `do_tar_bzip()`: to compress in bzip format
+- - `do_tar_lz4()`: to compress in lzip/lz4 format
+- `arch_translate()`: takes care of translating arch to find qemu static builds
+- `die()`: to echo an error message & exit if `ENABLE_EXIT` is set to true
+- `warn()`: like `die()` without exit
+- `shout()`:  for printing logs
+- `msg()`: for normal echo
+
+### Environment Variables
+- `ENABLE_EXIT` ( true | false ): to exit on error
+- `INCLUDE_PACKAGES`: to set extra packages to install in the bootstrap process
+- `SUITE`: to set target suite to bootstrap
+
+### MISC variable
+- `SUDO` (not user-defined) : if non-root user then value is path of sudo, if root no value/null string
+
+<hr>
+
+- just make sure not to mess up with existing function ( inform maintainers if any changes made in existing functions ) 👌