Update readme with 400 error info

[sn1020]

The latest release of codecov-python includes changes to resolve a specific 400 error while uploading to s3 (mentioned here). 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 #268 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #268   +/-   ##
=====================================
  Coverage      88%    88%           
=====================================
  Files           2      2           
  Lines           9      9           
=====================================
  Hits            8      8           
  Misses          1      1           

[Kami]

We started to encounter the same issue a couple of days ago and I can confirm the last version fixes it 👍

On that note, I believe it would be a good idea to update codecov binary to exit with non-zero status code on such failures, even if --required flag is not provided.