feat(array-selector): add subject

subjects/devops/array-selector/README.md

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+## array-selector
+
+### Instructions
+
+Create a script `array-selector.sh`, which will have an array declared with the
+following values (in this order): `red`, `blue`, `green`, `white`, `black`.
+
+When executed, the script will try to print the element in the position specified in the first argument passed to the script.
+
+The script should return `Error` when:
+
+- the number of argument passed are different from one,
+
+- the argument passed is not a number, 
+
+- the argument passed is a number out of the array range.
+
+Expected output:
+
+```console
+$ ./array-selector.sh 2
+blue
+$ ./array-selector.sh 5
+black
+$ ./array-selector.sh 6
+Error
+$ ./array-selector.sh 
+Error
+$
+```
+
+### Hints
+
+`array` is a special variable type that can hold more than one value. Below an example on how to define an array, select an element in the specific position, and how find out the length of an array. Below an example `script.sh`.
+
+```bash
+#!/usr/bin/env bash
+
+array=("one" "two" "three")
+
+echo ${array[0]} # display element in postion 0
+echo ${array[2]} # display element in position 2 
+echo ${#array[@]} # display length of array
+```
+
+And its output:
+
+```console
+$ bash script.sh
+one
+three
+3
+```
+
+[Bash conditional construct](https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs) can be used to decide whether to execute a specific command. Below an example `script.sh`.
+
+```bash
+#!/usr/bin/env bash
+
+if [[ 1 > 2 ]]; then
+    echo "true"
+else 
+    echo "false"
+fi
+```
+
+And its output:
+
+```console
+$ bash script.sh 
+false
+```
+
+It is possible to combine several conditions with the **AND** (`&&`) and **OR** (`||`) logical operators. Below and example `script.sh`.
+
+```bash
+if [[ 1 > 2 ]] || [[ 1 == 1 ]]; then
+    echo "true"
+else
+    echo "false"
+fi
+
+if [[ 1 > 2 ]] && [[ 1 == 1 ]]; then
+    echo "true"
+else
+    echo "false"
+fi
+```
+
+And its output:
+
+```console
+true
+false
+```