Documentation: Add FeeBumpTransaction code example to README

[sayali19425-gif]

Description

I was building a fee sponsorship (gasless transactions) feature
using js-stellar-sdk and found that the README doesn't have a
clear example of how to build a FeeBumpTransaction.

Suggested addition

Add a code example showing:

1. Building an inner transaction signed by user
2. Wrapping it in buildFeeBumpTransaction
3. Signing with sponsor keypair
4. Submitting to Horizon

Why this helps

Fee bump transactions are a powerful Stellar feature for
gasless UX but lack clear examples in the official docs.
This would help developers implement fee sponsorship faster.

Reference implementation

I implemented this in FlowFund:
https://github.com/sayali19425-gif/flowfund-level6