docs(file-checker): add description of subject

subjects/devops/file-checker/README.md

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+## file-checker
+
+### Instructions
+
+In this exercise you will make a script `file-checker.sh` that will check the status of a given file using if statements.
+
+Your script should make the following checks:
+
+- File exists.
+
+- File is readable.
+
+- File is writable.
+
+- File is executable.
+
+It should output the status as in the example in the usage.
+
+You should also handle what to do when no file is provided. The use of `test` command is not allowed for this exercise.
+
+### Usage
+
+```console
+$ ./file-checker.sh file.txt
+File exists
+File is readable
+File is not writable
+File is not executable
+$ ./file-checker.sh
+Error: No file provided
+$
+```
+
+### Hints
+
+[bash conditional expressions](https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html) can be used to solve this exercise.
+
+For example for `script.sh`:
+
+```bash
+#!/usr/bin/env bash
+
+if [ -s $1] ; then
+    echo "File size greater than 0"
+fi
+```
+
+And its output:
+
+```console
+$ ./script.sh file.txt
+File size greater than 0
+$
+```