Doc: add status badge for ncs upmerge check

add badge to see if there will be issues with building on  next upmerge

Signed-off-by: Robert Gałat <[email protected]>

README.rst

@@ -1,6 +1,8 @@
 .. image:: https://github.com/nrfconnect/sdk-sidewalk/actions/workflows/on-commit.yml/badge.svg?branch=main
  :target: https://github.com/nrfconnect/sdk-sidewalk/actions/workflows/on-commit.yml
 
+.. image:: https://github.com/nrfconnect/sdk-sidewalk/actions/workflows/verify-new-ncs.yml/badge.svg
+ :target: https://github.com/nrfconnect/sdk-sidewalk/actions/workflows/verify-new-ncs.yml
 
 nRF Connect SDK: sdk-sidewalk
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