docs: add Testnet vs Mainnet guide to README #124

[David-282]

Closes #124

Adds a new "Testnet vs Mainnet" section to the README explaining the key
differences between Stellar networks and how to switch between them using
StellarKit API's environment config.

Changes

• README.md — new section inserted after the FAQ section

Section covers

• Comparison table: real funds, network resets, Friendbot availability,
  Horizon URLs
• Exact .env values to change when switching networks
• Mainnet safety considerations warning block

Notes

• No code changes — documentation only
• HORIZON_URL is automatically derived from STELLAR_NETWORK and does
  not need to be set manually when switching networks

[drips-wave]

@David-282 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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