Added link to article about linux cli (issue #309)

[AsyaYeromina]

Please consider following material

[OleksiyRudenko]

@AsyaYeromina thank you for your proposal as well as for #324 and #326 that alltogether resolve #309

Please describe why these materials are helpful in terms what is it there that other materials, already available, do not have or disclose in a not so comprehensive manner.

Please note that every extra material extends time-to-complete for students. Time estimates (reading time, video duration etc) for those extra materials would be really helpful.

Thank you

[AsyaYeromina]

@OleksiyRudenko
The material added with this PR is offered as an additional.
Reading time is less than 30 minutes.

This article has the same information, or even less.
The material that is given on the course are interactive courses and video courses (which are good for assimilating knowledge)
But the article would be helpful for repeating what has been learned, or using it as a summary. When a student has forgotten a command, it would be much more difficult to find it in an interactive course or video than in such article.

Although you may disagree, referring the fact that in such cases it's better to look in official documentation.
In any case, it's at your discretion.
But for a variety of content types, I would add this as an additional material (or some other resources like this on w3c, or this on codecademy, or the command list on linuxsurvival)

[suchov]

Sorry habr...