Add Windows OpenCL instructions in the vignette

[rok-cesnovar]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Fixes #493

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Rok Češnovar

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[rok-cesnovar]

I included the .html, not sure if its required or not.

[codecov-commenter]

Codecov Report

Merging #602 (7de737a) into master (18c0f5d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #602   +/-   ##
=======================================
  Coverage   92.97%   92.97%           
=======================================
  Files          12       12           
  Lines        3262     3262           
=======================================
  Hits         3033     3033           
  Misses        229      229           

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[jgabry]

I just realized this was never merged, maybe because the macOS devel failure. Will rerun the checks now. I think this is ready otherwise, right?

[rok-cesnovar]

Yeah, this is good to go. Thanks!