[docs] Add avacus app links

[ludowkm]

Avacus wallet app supported Solana Pay, please help me to review and add the links to README, thank you!

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solana-pay-docs – ./docs

🔍 Inspect: https://vercel.com/solana-labs/solana-pay-docs/7Y7dBCSRorFCThhVnJeeTEoouRb8
✅ Preview: https://solana-pay-docs-git-fork-ludowkm-avacus-links-solana-labs.vercel.app

solana-pay – ./point-of-sale

🔍 Inspect: https://vercel.com/solana-labs/solana-pay/53uBBrJpR3agVTphu7MbtSgTwxEc
✅ Preview: https://solana-pay-git-fork-ludowkm-avacus-links-solana-labs.vercel.app

[ludowkm]

hi @jordansexton , please help me to review and add this links, thank you so much!

[ludowkm]

hi @benmills , Avacus is attempting to support Solana network as well as Solana Pay, please help us to merge this docs so that we can introduce both Solana Pay and Avacus, thank you!

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Name	Status	Preview	Updated
solana-pay-docs	✅ Ready (Inspect)	Visit Preview	May 9, 2022 at 1:33AM (UTC)