[Process] Add documentation about support of pipes as Process input

[romainneutron]

This issue is a reminder for myself and related to symfony/symfony#10934

[cordoval]

is this all because we are trying to support pipes on the builder 😊 ?

[romainneutron]

It's part of the idea, yes

[xabbuh]

@romainneutron Will you work on this or can others take it over?

[romainneutron]

Hello @xabbuh, feel free to propose a PR, no worry :)

[wouterj]

ping @xabbuh @romainneutron

[stof]

@wouterj this should be tagged as actionable and easy pick IMO

[xabbuh]

@stof good idea

@wouterj I didn't work on this yet. Asking @romainneutron was more a general question.