doc(readme): add LLVM install instructions for Arch Linux

[Wodann]

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[codecov]

Codecov Report

Merging #65 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   71.55%   71.52%   -0.03%     
==========================================
  Files         106      106              
  Lines        7203     7204       +1     
==========================================
- Hits         5154     5153       -1     
- Misses       2049     2051       +2

Impacted Files	Coverage Δ
crates/mun_codegen/src/ir/body.rs	86.29% <0%> (-0.47%)	⬇️
crates/mun_codegen/src/test.rs	100% <0%> (ø)	⬆️
crates/mun_codegen/src/ir/dispatch_table.rs	94.73% <0%> (ø)	⬆️
crates/mun_runtime/src/macros.rs	26.31% <0%> (ø)	⬆️

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