Edit readme to clarify that dev is the main develop branch

[nahuakang]

Description

This PR addresses Issue #194 and clarifies which branch is active under development.

Affected Dependencies

N/A

How has this been tested?

N/A

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev@16b63bc). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             dev     #234   +/-   ##
======================================
  Coverage       ?   79.88%           
======================================
  Files          ?       35           
  Lines          ?     1193           
  Branches       ?      160           
======================================
  Hits           ?      953           
  Misses         ?      215           
  Partials       ?       25           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 16b63bc...32b4f29. Read the comment docs.