Document heap size configuration

[flash1293]

Summary

This PR add an explanation to the readme how to increase the heap size limit to prevent OOM crashes during development. This got necessary recently because the project got too big.

Checklist

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• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

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[elasticmachine]

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[mattkime]

@elasticmachine update branch

[elasticmachine]

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[pmuellr]

LGTM - I've seen this advice, and contributed it in conversations as well, numerous times! Glad to finally have this in the doc.