Update acsf-setup.md with subprofile guidance instead of lightning.extend.yml

[vbouchet31]

https://github.com/acquia/blt/blob/8.x/readme/acsf-setup.md

https://docs.acquia.com/lightning/subprofile

[vbouchet31]

Previous:

• Execute blt acsf:init from the project root.
• Create a custom module in docroot/modules/custom that will be used in lieu of a custom profile.
• Add acsf as a dependency for your custom module.
• If using lightning, create a custom lightning.extend.yml and add it to sites/default
• Deploy to Cloud using blt deploy
• Use ACSF's "update code" feature to push the artifact out to sites.
• When creating a new site, select "Lightning" as the profile.

Suggested new:

• Execute blt acsf:init from the project root.
• Create a custom subprofile as described here.
• Add acsf as a dependency for your custom subprofile.
• Deploy to Cloud using blt deploy
• Use ACSF's "update code" feature to push the artifact out to sites.
• When creating a new site, select your custom subprofile as the profile.

[vbouchet31]

Probably need a step to edit the blt/project.yml to use the custom profile instead but it is not specific to ACSF.

[lcatlett]

Might want to add to this that code can be deployed via cloud hooks and that the above process will not work for updating existing acsf sites using lightning.

[grasmash]

@vbouchet31 Seems like you have a good idea of what needs to be done, could you submit a PR with your suggestions?

[malikkotob]

@vbouchet31, @lcatlett, mind taking a look at the PR? #1271

Feel free to leave suggestions/comments