content: Add historical scoring info and deprecations for Lighthouse 6.0

[kaycebasques]

Author: @kaycebasques
Reviewer: @paulirish

Tasks

• Call out deprecations
  • Addressed in Fix #1630 #1660
• Add historical scoring to the Lighthouse scoring doc
  • Addressed in Fix #1630 #1660

Target publish date: 2019-10-17 (Chrome 79 Branch Point)

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Process

Check out the web.dev writing process.

When you're ready, make a copy of this template
and fill in the proposal and outline sections. When you're finished, drop a link
to the doc in the 1st step below and check the box. A member of the web.dev team
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complete the remaining steps.

• 1. I've done my proposal
• 2. Proposal approved — reviewer checks this
• 3. I've added a draft to my proposal doc
• 4. Draft approved — reviewer checks this
• 5. I've submitted a pull request Fix #1630 #1660
• 6. Pull request approved — reviewer checks this

[paulirish]

@kaycebasques i have some WIP on this. Perhaps we can collab?

[carolrivero]

Please confirm who the reviewer is on this one. Thanks.

[kaycebasques]

@paulirish I wouldn't have it any other way. I'll write and you can review.

[kaycebasques]

@paulirish #1660 is my first pass, please leave feedback and then assign this issue back to me

P.S. I should've done the work on googlechrome/web.dev so that you could push edits, but didn't think about that, so I just gave you collab access to my fork so you should be able to push to kaycebasques/web.dev.

[kaycebasques]

Paul reviewed the PR 2 days ago, so next step is for me to incorporate feedback.