Document how to point the SDK at a custom contract and network

[Abioladory123]

Summary

networkFromEnv exists for supplying a custom RPC URL, passphrase and token
address, and the README's environment variable section lists the values. What is
missing is the short worked path a contributor actually needs: deploy your own
contract, point the SDK at it, and read a jar back.

That is the first thing anyone developing against this package locally has to
do, and it is currently assembled from three separate sections.

Acceptance criteria

• The README carries a worked example configuring the SDK against a
  self-deployed contract on testnet.
• It covers both the network configuration and the contract id together.
• The example has been run before the pull request is opened.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/novatip-sdk.git
cd novatip-sdk
git remote add upstream https://github.com/Novatip/novatip-sdk.git
npm install

Create a branch for this issue:

git checkout -b docs/custom-network-guide

Suggested commit message:

docs: document custom contract and network setup

Run npm run lint, npm run typecheck, and npm run build before pushing, then open a pull request from your fork against main and link this issue. See the contributing guide for the full workflow.

[glendell03]

Could you assign this to me?

networkFromEnv already handles the RPC URL, passphrase, and token address, and the README's environment variable section documents each value individually — but there's no single walkthrough tying network config to a specific contractId. I'd add a "Custom network and contract" section showing: deploying a contract to testnet, setting the corresponding env vars for networkFromEnv, instantiating the SDK with that contractId, and reading a jar back to confirm it works end-to-end.

One thing I'd rather confirm before writing: should the example use the Soroban CLI for deployment, or is there a preferred deploy script already in this repo? I'll run the example against testnet before opening the PR either way.