Improved tail.md
Merge pull request #167 from brunoduran/master

Improved tail.md
Added Linux and Jetbrains coverage to gitignore
Added useradd.md with most relevant use cases
Added a grep use case to ps
Merge pull request #169 from rubenvereecken/gitignore

Added Linux and Jetbrains coverage to gitignore
Merge pull request #170 from rubenvereecken/useradd

Added useradd.md with most relevant use cases
Merge pull request #171 from rubenvereecken/ps

Added a grep use case to ps