Improve docs continuous integration using Circleci

[saadaouad]

What is new on this PR?

Since i get this error in Circleci

+
TEST
Spin up Environment00:01
Checkout code00:00
npm install02:17
$(npm bin)/cypress run --record00:09
Exit code: 1
#!/bin/bash -eo pipefail
$(npm bin)/cypress run --record
npm info it worked if it ends with ok
npm info using npm@3.10.10
npm info using node@v6.11.1
npm info ok 
It looks like this is your first time using Cypress: 1.4.2

[01:25:47]  Verifying Cypress can run /var/www/node_modules/cypress/dist/Cypress [started]
[01:25:49]  Verifying Cypress can run /var/www/node_modules/cypress/dist/Cypress [completed]

Opening Cypress...
You passed the --record flag but did not provide us your Record Key.

You can pass us your Record Key like this:

  cypress run --record --key <record_key>

You can also set the key as an environment variable with the name CYPRESS_RECORD_KEY.

https://on.cypress.io/how-do-i-record-runs
Exited with code 1

I think it's better to add --key <record_key> in the docs

[CLAassistant]

All committers have signed the CLA.

[saadaouad]

FYI @jennifer-shehane

[bahmutov]

👍 thanks @saadaouad

[saadaouad]

Sure @bahmutov