doc: contd documentation

Author: RawanMostafa08

docs/cmds/contd.md

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+# Contd
+
+Container module is reponsible for handling starting, stoppping and inspecting of containers.
+
+Contd provides the entrypoint of the contd module, exposing it as a service over the message bus (zbus) so that other modules can handle the containers.
+
+## How it works
+
+- Ensure that `shim-logs` binary is available, backoff until it's found.
+- Creates module root dirctory.
+- Connects to `Redis` message broker over `zbus`.
+- Register itself as a `containerd` module.
+- Starts watching for events coming from `containerd`.
+
+
+## Usage
+
+```sh
+contd --root /var/cache/modules/contd \
+>    --broker unix:///var/run/redis.sock \
+>    --congainerd /run/containerd/containerd.sock \
+>    --workers 2
+```
+
+### Command-line flags
+
+| Flag              | Description                                     | Default                           |
+| -----------       | ------------------------------------------------| ----------------------------      |
+| `--root`          | `ROOT` working directory of the module          | `/var/cache/modules/contd`        |
+| `--broker`        | Connection string to the message BROKER         | `unix:///var/run/redis.sock`      |
+| `--congainerd`    | connection string to containerd `CONTAINERD`    | `/run/containerd/containerd.sock` |
+| `--workers`       | Number of workers N                             | `1`                               |