Improve discoverability of supported syntax

[rawnsley]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Small tweak to readme to make it clearer where to find the full list of supported directive syntax.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e1027d1) to head (ad69c7f).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #22   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           40        40           
=========================================
  Hits            40        40           

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[wooorm]

I appreciate wanting to help but I have doubts about this. More words is not always better. Succinct headings is also nice. And, consistency across all the projects: most remark plugins / syntax-tree utilities / micromark extensions have these headings!