Update README.md

[v-thepet]

Updating language to reflect that the transition from Azure RTOS to Eclipse ThreadX has already happened and is complete.

[timlt]

Updates look good for the completed transition RTOS > ThreadX.

[fdesbiens]

@v-thepet We are ready to merge this PR. However, we can only do so once it passes our contribution agreement check.

You need an Eclipse Foundation account under the same email address used to sign your commits. Also, your Eclipse account must specify Microsoft as your employer so that the check will pass. You do not need to sign the contribution agreement individually since you are covered by the Member Committer and Contributor Agreement (MCCA) filled by Microsoft.

[v-thepet]

@fdesbiens OK I set up an account, let me know if there's anything more, thanks.