Add Solana Hardware requirement to README

[lubao]

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What is the outcome that you are trying to reach?

Add Solana Hardware requirement to README to prevent out of memory error for different type or node.

Describe the solution you would like

Add a section point to https://docs.solanalabs.com/operations/requirements to clarify HW requirement for different node type.

Describe alternatives you have considered

Additional context

[frbrkoala]

Added new configuration options after additional testing. The older instance types seems no longer sufficient, so switched to R7a.