docs(README): add header-only readme

[MeteorsLiu]

No description provided.

[qiniu-x]

[Git-flow] Hi @MeteorsLiu, There are some suggestions for your information:

---

Rebase suggestions

• Following commits seems generated via git merge

  Merge branch 'main' of https://github.com/goplus/llcppg into docs/add-headeronly-readme

Which seems insignificant, recommend to use git rebase command to reorganize your PR.

For other git-flow instructions, recommend refer to these examples.

Details

If you have any questions about this comment, feel free to raise an issue here:

• https://github.com/qiniu/reviewbot

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.14%. Comparing base (3e8fd41) to head (789b260).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #496   +/-   ##
=======================================
  Coverage   84.14%   84.14%           
=======================================
  Files          27       27           
  Lines        2738     2738           
=======================================
  Hits         2304     2304           
  Misses        391      391           
  Partials       43       43           

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