docs: add local Foundry dev + test quickstart (by @sadosist)

[sadosist]

Overview

Adds a small “Local development (Foundry quickstart)” section to the README to reduce first-time setup friction.

Description

The new block includes:

• Foundry install
• build/test commands
• local anvil chain
• optional deploy script
• useful cast checks

Docs-only; no code changes.

Testing instructions

Run on a fresh env:

curl -L https://foundry.paradigm.xyz | bash
foundryup
forge install
forge build
forge test -vv
anvil --chain-id 31337
# (optional) deploy:
forge script script/Deploy.s.sol \
  --rpc-url http://127.0.0.1:8545 \
  --private-key <ANVIL_PRIVATE_KEY> \
  --broadcast
cast chain && cast block-number

[sadosist]

Added a minimal Foundry local-dev quickstart to README.
Happy to adjust naming/placement or move it under docs/LOCAL_DEV.md.
I can also follow up with a tiny resolver script + CI hints if helpful. — @sadosist