Document how to speed up full validation in the README

[teor2345]

Motivation

We want to document how to speed up Zebra in the README, to help with issues like #3042.

Tasks

Document additional network requirements:

• must have some peers with a round-trip latency of 4 seconds or less

zebra/zebra-network/src/constants.rs

Line 54 in f6de7fa

pub const HANDSHAKE_TIMEOUT: Duration = Duration::from_secs(4);

Explain how to speed up full validation in the README:

• reduce the lookahead limit - always speeds things up
• reduce the peerset target size - can slow things down if it's too low - try 5, 15, and 25
• reduce the number of concurrent block requests - can slow things down if it's too low - try 1, 5, and 15
• use a release build (default for cargo install)
• log at info level (default config)
• build for the native processor (not a VM or Rosetta on Apple M1)

[sync]
max_concurrent_block_requests = 25
lookahead_limit = 800