Update readme with 400 error info

[sn1020]

Latest release of codecov-node includes changes to resolve a specific 400 error while uploading to s3. This PR updates the readme to encourage users to make sure they have the latest version of the package if they encounter 400 errors during upload.
While they could be running into 400 errors for an entirely different reason than those addressed in the latest release, updating and seeing if that resolved the issue is generally a good first step to try here.
Happy to change the wording around, just let me know what y'all would prefer.

[codecov]

Codecov Report

Merging #181 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   89.66%   89.66%           
=======================================
  Files          23       23           
  Lines         358      358           
  Branches       87       87           
=======================================
  Hits          321      321           
  Misses         37       37           

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