[#796] Add help for Strict Mode in README

[stefcameron]

Fixes #796

PR Checklist

Please leave this checklist in your PR.

• Issue being fixed is referenced.
• Source changes maintain:
  • Stated browser compatibility.
  • Stated React compatibility.
• Unit test coverage added/updated.
• E2E test coverage added/updated.
• Prop-types added/updated.
• Typings added/updated.
• Changes do not break SSR:
  • Careful to test typeof document/window !== 'undefined' before using it in code that gets executed on load.
• README updated (API changes, instructions, etc.).
• Changes to dependencies explained.
• Changeset added (run yarn changeset locally to add one, and follow the prompts).
  • EXCEPTION: A Changeset is not required if the change does not affect any of the source files that produce the package bundle. For example, demo changes, tooling changes, test updates, or a new dev-only dependency to run tests more efficiently should not have a Changeset since it will not affect package consumers.

[changeset-bot]

🦋 Changeset detected

Latest commit: 22e1dce

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
focus-trap-react	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[codecov]

Codecov Report

Base: 90.32% // Head: 90.32% // No change to project coverage 👍

Coverage data is based on head (66db23b) compared to base (cb8d098).
Patch has no changes to coverable lines.

❗ Current head 66db23b differs from pull request most recent head 22e1dce. Consider uploading reports for the commit 22e1dce to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #804   +/-   ##
=======================================
  Coverage   90.32%   90.32%           
=======================================
  Files           1        1           
  Lines         155      155           
  Branches       68       68           
=======================================
  Hits          140      140           
  Misses         13       13           
  Partials        2        2           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.