misc: add requirements for sites being added to showcase

[lex111]

Motivation

I think these paragraphs will help tell people what sites we want to see in the showcase.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

N/A.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

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Size Change: +87 B (0%)

Total Size: 156 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	17.6 kB	+34 B (0%)
website/build/assets/js/main.********.js	109 kB	+33 B (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	21.7 kB	+11 B (0%)
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	6.95 kB	+9 B (0%)

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Deploy preview for docusaurus-2 ready!

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	63
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4210--docusaurus-2.netlify.app/classic/

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	90
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4210--docusaurus-2.netlify.app/classic/

[netlify]

Deploy preview for docusaurus-2 ready!

Built with commit 0e29fd3

https://deploy-preview-4210--docusaurus-2.netlify.app

[slorber]

Thanks, just updated some wordings to fix grammarly feedbacks + also set the bar lower for the "minimal customization" part which may be a bit unclear.

I'd like this page to have a lot of sites in the future, and will ask MLH fellows to build some kind of search engine with tags so that we can easily see Facebook sites, or sites with highly customized designs etc...