message: Add program magic comment related readme contents
diff: @@ -123,6 +123,27 @@ To find the root file, LaTeX Workshop will follow the steps below, stopping when
  If no root file is found, most of the features in LaTeX Workshop will not work.
  +### Magic comments?
  +LaTeX Workshop supports both `% !TEX root` and `% !TEX program` magic comments. The former is used to define the root file, while the latter helps select compiler program.
  +
  +All `command` in `toolchain` which are empty will be replaced with the program set by `% !TEX program` magic comment in the root file. Suppose there is a line `% !TEX program = xelatex` in the root file, and the toolchain is set as follows:
  +```
  +"latex-workshop.latex.toolchain": [
  + {
  + "command": "",
  + "args": [
  + "-synctex=1",
  + "-interaction=nonstopmode",
  + "-file-line-error",
  + "%DOC%"
  + ]
  + }
  +]
  +```
  +Upon building the project, LaTeX Workshop will parse the root file and figure out that `xelatex` should be used. This program will replace the empty `command` in the toolchain. Arguments are untouched.
  +
  +If the `command` is set empty but no `% !TEX program` magic comment is found, `pdflatex` is used.
  +
  ### Spell check?
  [Code Spellchecker](https://marketplace.visualstudio.com/items?itemName=streetsidesoftware.code-spell-checker) did a great job. The following regexps are recommended to be ignored for LaTeX:
  ```
  