Add README for checker

Add instructions on using the checker: local, remote, Moodle.

Signed-off-by: Razvan Deaconescu <razvan.deaconescu@upb.ro>

README.checker.md

+# Using the Checker
+
+Use the automated infrastructure to check and grade the assignment.
+This uses a Docker container to build the solution and to build and run the tests.
+Using a Docker container provides two benefits:
+
+1. it uses a given stable setup;
+   no need to worry about different versions
+
+1. it provides an isolated environment that can be easily created and destroyed
+
+The Docker container and the checking can be done:
+
+- **locally**, through the use of the `local.sh` script
+- **remote** using GitLab pipelines, by doing a push in the remote repository
+
+Entries in the repository:
+
+- `checker/`: the high-level checker script (`checker.sh`)
+- `tests/`: actual tests used to grade the assignment, called by `checker.sh`
+- `src/`: initial assignment skeleton that is to be filled with the solution
+- `Dockerfile`: used to construct the container used to build and test the assignment
+- `local.sh`: script for local management of the Docker container, also for pushing it to the remote registry
+- `CPPLINT.cfg`, `.shellcheckrc`, `.checkpatch.conf`: linter configuration files
+
+Full instructions on using the checker are part of the [VMChecker Student Handbook](https://github.com/systems-cs-pub-ro/vmchecker-next/wiki/Student-Handbook).
+
+## Setting Up
+
+In order to set up the repository, do the following:
+
+1. Login into your GitLab account using the UPB login at https://gitlab.cs.pub.ro/users/sign_in
+
+1. Make a **private** fork of this repository.
+   In the right corner you can find the fork button.
+   After clicking it, the following should be prompted at you:
+
+   ![private_fork](./img/private_fork.png)
+
+   Select your user as the namespace for your fork.
+   Make sure the visibility level is set to **Private**.
+
+1. Clone the repository locally:
+
+   ```
+   $ git clone <remote_of_your_private_fork>
+   ```
+
+Now you can work on the assignment and then test it.
+
+## Working on the Assignment
+
+To work on the assignment follow the instructions in `README.md`.
+You need to enter the `src/` directory and update the source code files.
+
+While working on the assignment, run the checker to see if certain tests work, and they are graded.
+
+Do frequent Git commits with your changes to make sure you have a history of your work.
+
+## Local Checker
+
+To run the checker locally inside a Docker container, use the `local.sh` script.
+
+First, build the container:
+
+```console
+$ ./local.sh docker build
+[2023-03-28T19:59:52] [INFO] Building Docker image...
+[...]
+Successfully tagged gitlab.cs.pub.ro:5050/operating-systems/assignment-[...]:latest
+```
+
+And now run the checker inside the container:
+
+```console
+$ ./local.sh checker
+```
+
+Note that checking typically consists of two steps:
+
+1. Run the actual tests in the `tests/` directory and grade the assignment.
+
+1. Run linter steps on the solution (and tests) and provide an output.
+   For maximum grade, assignments must remove all warnings and errors from linters.
+
+## Remote Checker
+
+To use the remote checker, push your changes into your fork.
+Each time you do a fork, a remote GitLab pipeline will run a checker job.
+
+Visit the fork on the [GitLab web interface](https://github.cs.pub.ro).
+Access the `CI/CD -> Jobs` entry in the left menu.
+You would get a view such as the one in the image below:
+
+![GitLab CI/CD Job](img/gitlab-jobs.png)
+
+Click on the status of the job to see the run results.
+The results are the ones you get with the local checker: a checker output and a linter output.
+
+Each time you do a push, a new job will be created.
+
+## Moodle Submission and Grading
+
+To grade the assignment, it must through pass through [Moodle](https://curs.upb.ro).
+This can be done through either a direct submission through Moodle or by picking up pushed content in the fork.
+
+After this is done, the results of the submission will be available as feedback comments on Moodle.
+Also, the grade from the submission will be made available on Moodle.
+
+Results in Moodle are presented similar to the image below:
+
+![Moodle Results](img/moodle-results.png)
+
+### Direct Moodle Submission
+
+A direct Moodle submission will use the Moodle interface to submit an archive.
+
+For this:
+
+1. Create an archive.
+   Do this by entering the `src/` directory and running:
+
+   ```console
+   $ make pack
+   ```
+
+   A file called `src.zip` will be created.
+
+1. Access the assignment submission entry on Moodle and submit the archive there.
+   After submitting, a GitLab run will be triggered with VMChecker.
+   In a matter of minutes, results and grade will be available.
+
+### Pushed Content Pickup
+
+You can also directly pick up the results of a Git push submission using Moodle.
+
+For this, you would have to use the Moodle block for the assignment, as instructed [here](https://github.com/systems-cs-pub-ro/vmchecker-next/wiki/Student-Handbook#32-submitting-an-assignment).
+
+You need to fill the items:
+
+- `Access token`: [Generate a personal access token](https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html#create-a-personal-access-token)
+- `Project ID`: the ID of the project in GitLab
+- `Branch`: the Git branch that was used and pushed on the GitLab fork
+
+After filling the items and submitting, a GitLab job will be stared and, then, the results will be made available on Moodle.
+
+
+Recall that full instructions on using the checker are part of the [VMChecker Student Handbook](https://github.com/systems-cs-pub-ro/vmchecker-next/wiki/Student-Handbook).