Update README.md with new URL link to Zigzag

[Hedda]

Update README.md with new URL link to Zigzag

https://github.com/Samantha-uk/zigzag-v1

[coveralls]

Coverage remained the same at 99.815% when pulling aead8c2 on Hedda:patch-33 into e837338 on zigpy:dev.

[codecov-io]

Codecov Report

Merging #546 (aead8c2) into dev (e837338) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #546   +/-   ##
=======================================
  Coverage   99.81%   99.81%           
=======================================
  Files          41       41           
  Lines        5416     5416           
=======================================
  Hits         5406     5406           
  Misses         10       10           

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