chore: multichain-node update README.md and add .nvmrc

[ffmcgee725]

Explanation

This PR updates documentation on multichain-node playground package to make sure the correct node version is used when locally running the project.

References

• Fixes https://www.notion.so/metamask-consensys/SDK-Phase-1-End-game-276f86d67d688061a461ee0cbaf37649?p=287f86d67d688045bff7e300b2078033&pm=s

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

---

Note

Adds .nvmrc (v20.19.0) and updates README to instruct using nvm before setup.

• Playground multichain-node:
  • Config: Add .nvmrc specifying Node v20.19.0.
  • Docs: Update README.md to include nvm use instructions prior to yarn install.

^{Written by Cursor Bugbot for commit f18a3a0. This will update automatically on new commits. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.99%. Comparing base (c2cb539) to head (f18a3a0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1367   +/-   ##
=======================================
  Coverage   74.99%   74.99%           
=======================================
  Files         184      184           
  Lines        4519     4519           
  Branches     1108     1108           
=======================================
  Hits         3389     3389           
  Misses       1130     1130           

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud