message: additional edit in viewer/README.md
diff: @@ -4,7 +4,9 @@ The pdf viewer is based on [pdf.js](https://mozilla.github.io/pdf.js/) by Mozill
  Mozilla [asks](https://mozilla.github.io/pdf.js/getting_started/) web developers to reskin `viewer.html` because Firefox users would think bugs of the viewer on the web site are ones of Firefox and would report them to the pdf.js team. See [link](https://github.com/mozilla/pdf.js/issues/5609). Our usage does not cause such a problem.
  -You can see the [changes](https://github.com/James-Yu/LaTeX-Workshop/compare/a0b97a9...a5f8c04#diff-ff661e0ff756ae1ff026c0e8f4561d0e) we have made to `viewer.js`. We had better find a way to achieve this without modifying `viewer.js`.
  +We provide additional features by setting up new event listeners in `latexworkshop.js` for DOM objects in `viewer.html`. We do not and should not override functions defined by pdf.js.
  +
  +We can see the [changes](https://github.com/James-Yu/LaTeX-Workshop/compare/a0b97a9...a5f8c04#diff-ff661e0ff756ae1ff026c0e8f4561d0e) we have made to `viewer.js`. We had better find a way to achieve this without modifying `viewer.js`.
  ### refreshing operation
  