Update README to remove ZenHub mentions

[LexaMichaelides]

Summary
Update README to remove mentions of ZenHub as that tool is no longer needed for onboarding.

Good news -  we no longer need ZenHub for onboarding - all of our issue management runs directly through GitHub Issues (I should probably make a note to update the README). [from Matt King on Slack]

Work Detail
Text-only change in README file

• Frontend: n/a
• Backend: n/a

Current State
Current README located here contains links to ZenHub and references ZenHub as part of the onboarding path.

Resources
n/a

[LexaMichaelides]

I can do this :)