docs: add a "code-style LLVM" badge

[aloisklink]

Adds the following badge to the README.md to show that we follow the LLVM code style:

[codecov]

Codecov Report

Merging #241 (2a8ab01) into main (474f7be) will increase coverage by 0.96%.
The diff coverage is n/a.

❗ Current head 2a8ab01 differs from pull request most recent head aa60f50. Consider uploading reports for the commit aa60f50 to get more accurate results

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
+ Coverage   44.64%   45.61%   +0.96%     
==========================================
  Files         110      107       -3     
  Lines       16393    14951    -1442     
==========================================
- Hits         7319     6820     -499     
+ Misses       9074     8131     -943     

Impacted Files	Coverage Δ
src/ap/hostapd.c	68.57% <0.00%> (-7.21%)	⬇️
src/utils/os.c	40.00% <0.00%> (-4.81%)	⬇️
tests/ap/test_hostapd.c	93.10% <0.00%> (-1.82%)	⬇️
src/utils/iface.c	18.18% <0.00%> (-1.60%)	⬇️
tests/dhcp/test_dnsmasq.c	93.46% <0.00%> (-0.05%)	⬇️
tests/utils/test_os.c	98.01% <0.00%> (-0.02%)	⬇️
src/config.c	0.00% <0.00%> (ø)
src/edgesec.c	0.00% <0.00%> (ø)
tests/dns/test_mdns_list.c	100.00% <0.00%> (ø)
tests/dns/test_mdns_mapper.c	100.00% <0.00%> (ø)
... and 10 more

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