Add note about std and alloc features to readme

[prestwich]

Motivation

They were missing from the large feature list

Solution

Separate note about the special features

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.01% 🎉

Comparison is base (32d5178) 82.73% compared to head (6ce2fb8) 82.75%.
Report is 2 commits behind head on main.

❗ Current head 6ce2fb8 differs from pull request most recent head abe9e4b. Consider uploading reports for the commit abe9e4b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
+ Coverage   82.73%   82.75%   +0.01%     
==========================================
  Files          52       52              
  Lines        5648     5648              
==========================================
+ Hits         4673     4674       +1     
+ Misses        975      974       -1     

see 3 files with indirect coverage changes

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