Inform user how to exit pipeline ascii display

[pared]

Context:
https://discordapp.com/channels/485586884165107732/563406153334128681/603890677176336394

https://dvc.org/doc/commands-reference/pipeline-show#options should probably contain note that q can be used to exit from dvc pipeline show --ascii

[ryokugyu]

@pared we just need to mention that press q to exit the ASCII display?

[efiop]

Also related to iterative/dvc#1208

[efiop]

@ryokugyu Ideally we should mention all the keys: https://github.com/iterative/dvc/blob/0.53.1/dvc/dagascii.py#L126 . arrows(or WASD) for navigation, q for exit.

[shcheklein]

@efiop ideally we should put this instruction to the DVC itself as well. Is it possible? It's too confusing for people who are not familiar with CLI that much.

[efiop]

@shcheklein Yep, there is an issue that i've linked above.

[shcheklein]

@efiop thanks!