Add --project flag to cypress run doc

[jennifer-shehane]

This flag is missing in docs. node_modules/.bin/cypress run --project ./src/clients/foobar

[brian-mann]

I purposely left this out because this only applies when Cypress is installed in global mode and even then we automatically set --project to process.cwd().

It's just a confusing internal flag that really serves no usable purpose (except for us). You would never be using cypress run outside of the project you want to run.

[brian-mann]

It is in cypress --help so power users could still use it when available.

[brian-mann]

I see there is a use case for this flag here: cypress-io/cypress#635

Now that I understand why someone would want to do this: we'll document that specific use case.