refine readme

v0.3

doc: describe action-usage and local usage

Convert markdown to a standalone single-file.

• Assets used by a markdown are pushed to a branch and the urls in the markdown are replaced.

  Assets are stored in a branch specified by inputs.asset_branch, by default
  _md2zhihu/asset.

  It pushes converted markdowns to a branch specified by inputs.md_branch,
  by default _md2zhihu/md.
  To retrieve the converted markdowns, a user fetches and merges this branch.

• Latex math such as $$ \frac{x}{y} $$ are converted into images.