Add a README file for system tests

#68

test/system/README.md

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+# System tests
+
+These tests are built with BATS (Bash Automated Testing System). They are
+strongly influenced by libpod project.
+
+The tests are meant to ensure that toolbox's functionality remains stable
+throughout updates of both Toolbox and Podman/libpod.
+
+## Structure
+
+- **Basic Tests**
+  - [] output version number (Toolbox + Podman)
+  - [x] show help screen when no command is given
+  - [x] create the default container
+  - [x] create a container with a custom name
+  - [x] create a container from a custom image
+  - [x] list containers (no present)
+  - [x] list default container and default image
+  - [x] list containers (some present; different name patterns)
+  - [x] list images (no present)
+  - [x] list images (some present; different name patterns)
+  - [x] remove a specific container
+  - [x] try to remove nonexistent container
+  - [x] try to remove a running container
+  - [x] remove all containers
+  - [x] try to remove all containers (running)
+  - [x] remove a specific image
+  - [x] remove all images
+  - [x] run a command inside of an existing container
+
+- **Advanced Tests**
+  - [ ] create several containers with various configuration and then list them
+  - [ ] create several containers and hop between them (series of enter/exit)
+  - [ ] create a container, enter it, run a series of basic commands (id,
+        whoami, dnf, top, systemctl,..)
+  - [ ] enter a container and test basic set of networking tools (ping,
+        traceroute,..)
+
+The list of tests is stil rather basic. We **welcome** PRs with test
+suggestions or even their implementation.
+
+## Convention
+
+- All tests that start with *Try to..* expect non-zero return value.
+
+## How to run the tests
+
+Make sure you have `bats` and `podman` with `toolbox` installed on your system.
+
+- Enter the toolbox root folder
+- Invoke command `bats ./test/system/` and the test suite should fire up
+
+By default the test suite uses the system versions of `podman` and `toolbox`.
+
+If you have a `podman` or `toolbox` installed in a nonstandard location then
+you can use the `PODMAN` and `TOOLBOX` environmental variables to set the path
+to the binaries. So the command to invoke the test suite could look something
+like this: `PODMAN=/usr/libexec/podman TOOLBOX=./toolbox bats ./test/system/`.