rootfs: Add README

Add docs about how rootfs generation.

Signed-off-by: Jose Carlos Venegas Munoz <jose.carlos.venegas.munoz@intel.com>

rootfs-builder/README.md

@@ -0,0 +1,126 @@
+# Building a rootfs for Kata Containers Guest OS #
+
+The Kata Containers `rootfs` is created using `rootfs.sh`.
+
+## Supported base OSs ##
+
+The `rootfs.sh` script builds a `rootfs` based on a particular Linux\*
+distribution. To build a `rootfs`for your chosen distribution, run:
+
+```
+$./rootfs.sh <distro>
+```
+
+To check the supported `rootfs` based OS run `$rootfs-builder/rootfs.sh
+-h`, it will show the supported values of `<distro>`
+
+
+## Adding support for new base OS ##
+
+The script `rootfs.sh` will it check for immediate sub-directories
+containing the following expected files structure:
+
+- A `bash(1)` script called `rootfs_lib.sh`
+
+  This file must contain a function called `build_rootfs()` this function
+  must receive as first argument the path where the `rootfs` will be
+  populated. Path: `rootfs-builder/<distro>/rootfs_lib.sh`.
+
+
+- A `bash(1)` file `config.sh`
+
+  This represents the specific configuration for `<distro>`. It must
+  provide configuration specific variables for user to modify as needed.
+  The `config.sh` file will be loaded before executing `build_rootfs()` to
+  provide all the needed configuration to the function. Path:
+  `rootfs-builder/<distro>/config.sh`.
+
+To create a directory with the expected file structure run:
+
+```
+make -f template/Makefile  ROOTFS_BASE_NAME=my_new_awesome_rootfs
+```
+
+After run the command above, a new directory will be created in
+`rootfs-builder/my_new_awesome_rootfs/`. To verify it is one of the
+options to build a `rootfs` run `./rootfs.sh -h`, it will show
+`my_new_awesome` as one of the options to use it for:
+
+```
+./rootfs.sh <distro>
+```
+
+Now that a new directory structure was created is need to:
+
+- If needed , add configuration variables to `rootfs-builder/my_new_awesome_rootfs/config.sh`
+- Implement the stub `build_rootfs()` function from `rootfs-builder/my_new_awesome_rootfs/rootfs_lib.sh`
+
+### Expected `rootfs` directory content ###
+
+After the function `build_rootfs` is called, the script expects the
+`rootfs` directory to contain /sbin/init and /sbin/kata-agent binaries.
+
+### (optional) Customise the `rootfs` ###
+
+For development uses cases, developers may want to modify the guest OS.
+To do that it is possible to use following methods:
+
+- Use the environment variable `EXTRA_PKG` to provide a list of space
+  separated packages to be installed.
+
+  *Note: The package names may vary among Linux* distributions, the extra
+  package names must exist in the base OS flavor you use to build the
+  `rootfs`*
+
+  Example:
+  ```
+  EXTRA_PKG="vim emacs" ./rootfs-builder/rootfs.sh \
+  -r ${PWD}/myrootfs fedora
+
+  ```
+
+- In `rootfs-builder/<distro>/config.sh` modify the variable `PACKAGES`.
+  This are the minimal set of packages needed. The configuration file must
+  use the package names from the distro was created for.
+
+- It is possible to customise the `rootfs` directory before create an
+  image based in on it.
+
+
+## Build `rootfs` using Docker* ##
+
+Depending on the base OS to build the `rootfs` guest OS, it is required some
+specific programs that probably are not available or installed in the system
+that will build the guest image. For this case `rootfs.sh` can use
+a Docker\* container to build the `rootfs`. The following requirements
+must be met:
+
+1. Docker 1.12+ installed
+
+2. `runc` is configured as the default runtime
+
+   To check if `runc` is the default runtime:
+
+   ```
+   $ docker info | grep 'Default Runtime: runc'
+   ```
+
+   Note:
+   This requirement is specifically when using Clear Containers runtime
+   see [issue](https://github.com/clearcontainers/runtime/issues/828) for
+   more information.
+
+3. Export `USE_DOCKER` variable
+
+   ```
+   $ export USE_DOCKER=true
+   ```
+4. Use `rootfs.sh:
+   Example:
+   ```
+   $ export USE_DOCKER=true
+   $ # build guest O/S rootfs based on fedora
+   $ ./rootfs-builder/rootfs.sh -r "${PWD}/fedora_rootfs" fedora
+   $ # build image based rootfs created above
+   $ ./image-builder/image_builder.sh "${PWD}/fedora_rootfs"
+   ```