Add our Developer Experience Engineer career opportunity to the README

[thinkingserious]

Issue Summary

The role of Developer Experience Engineer just became available and we want to announce it in the README. Here is the copy:

If you're a software engineer who is passionate about #DeveloperExperience and/or #OpenSource, this is an incredible opportunity to join our #DX team as a Developer Experience Engineer and work with @thinkingserious and @aroach! Tell your friends :)

Acceptance Criteria

• The above announcement is added to the Announcements section of this README

[luciajimenez]

I'm taking this one.

[mptap]

@thinkingserious Have raised PR for this: #89