Update README with ES6 import example.

[BigBlueHat]

Add ES6 import syntax for jsonld in README.

The other import styles will not work with Node.js, so this marks out the happy path for Node devs.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 93.71%. Comparing base (19d55bd) to head (909c23f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #576   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files          24       24           
  Lines        2992     2992           
=======================================
  Hits         2804     2804           
  Misses        188      188           

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 19d55bd...909c23f. Read the comment docs.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• :package: JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[davidlehn]

• Related: https://github.com/digitalbazaar/jsonld.js/pull/555.
• The ESM/CJS style mixing can get tricky sometimes. I was hoping to avoid mentioning it at all and move to ESM where we can control it better.