doc: add a README for LLL syntax and semantics

[charles-cooper]

What I did

How I did it

How to verify it

View it at https://github.com/charles-cooper/vyper/blob/docs/lll/vyper/lll/README.md

Description for the changelog

Document Vyper's current IR - syntax and semantics.

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[codecov-commenter]

Codecov Report

Merging #2494 (31e1079) into master (93a9579) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 31e1079 differs from pull request most recent head 3824183. Consider uploading reports for the commit 3824183 to get more accurate results

@@            Coverage Diff             @@
##           master    #2494      +/-   ##
==========================================
- Coverage   84.60%   84.59%   -0.01%     
==========================================
  Files          93       93              
  Lines        9228     9238      +10     
  Branches     2086     2088       +2     
==========================================
+ Hits         7807     7815       +8     
- Misses        935      936       +1     
- Partials      486      487       +1     

Impacted Files	Coverage Δ
vyper/builtin_functions/functions.py	88.95% <0.00%> (-0.35%)	⬇️
vyper/cli/vyper_lll.py	0.00% <0.00%> (ø)
vyper/lll/optimizer.py	84.07% <0.00%> (ø)
vyper/lll/compile_lll.py	94.49% <0.00%> (+0.14%)	⬆️

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[fubuloubu]

I know this is valid use of scoping, but for clarity it would be best not to re-use x in the with label here