Add a note about Online Editors.

[gokatz]

We can add a note about the flag FASTBOOT_DISABLED=true to disable Fastboot while working on any Online Editors such as https://codesandbox.io, https://gitpod.io etc., as Fastboot has a domain restriction by default and these online editors will serve the application on an arbitrary URL on the web (not on localhost)

Any alternate suggestion for this use case would also be great. Thanks.

(This was discussed in this PR on CLI Guides)