message: Improve the requirement of latexmk in README
  This closes
diff: @@ -35,7 +35,7 @@ One million downloads! This project won't be successful without contributions fr
  - LaTeX distribution in system PATH. For example, [TeX Live](https://www.tug.org/texlive/).
  - Please note [MikTeX](https://miktex.org/) does not ship with SyncTeX. See [this link](http://tex.stackexchange.com/questions/338078/how-to-get-synctex-for-windows-to-allow-atom-pdf-view-to-synch#comment877274_338117) for a possible solution.
  -- _Optional_: [Set your LaTeX recipe](#recipe) (LaTeX Workshop should just work out of the box for users with `latexmk` installed).
  +- `latexmk` is required for the default recipe for building LaTeX projects to work. Alternatively, you can [set up your own LaTeX recipe](#recipe).
  - _Optional_: Install [ChkTeX](http://www.nongnu.org/chktex) to lint LaTeX projects.
  - _Optional_: Install [latexindent.pl](https://github.com/cmhughes/latexindent.pl) for formatting support if it is not provided by your LaTeX distribution.
  