docs: mention v4 compatibility

[bard]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update
• documentation fix

Motivation / Use-Case

Following the instructions at https://v4.webpack.js.org/loaders/expose-loader/ and then https://github.com/webpack-contrib/expose-loader/ leads to a non-functional configuration for v4 users and, unless one is familiar enough with WebPack APIs to interpret the error message, no clue why.

This small change should give a just-in-time heads-up.

Breaking Changes

None.

Additional Info

[codecov]

Codecov Report

Merging #128 (5067895) into master (3bd8e6e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #128   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files           4        4           
  Lines         129      129           
  Branches       41       41           
=======================================
  Hits          119      119           
  Misses          7        7           
  Partials        3        3           

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[alexander-akait]

Please accept CLA

[bard]

@alexander-akait done. Note that no bot posted the link (as per https://github.com/webpack-contrib/expose-loader/blob/master/.github/CONTRIBUTING.md#contributor-license-agreement), I had to dig it out of CI checks.