servo: Merge #18353 - add note on windows build (from tigercosmos:readme-w); r=larsbergstrom

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add note on windows build to `README.md`
based  on #18291
@larsbergstrom, please check, thanks.
---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because `README`

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Source-Repo: https://github.com/servo/servo
Source-Revision: 7cb47f16361cee1a65edfa760510a07833ef689f

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 49a9316a8c47a613dd863487010ad262624063f4

servo/README.md

@@ -112,6 +112,12 @@ settings for the installer are fine).
 
 4. Install Visual Studio Community 2017 (https://www.visualstudio.com/). You MUST add "Visual C++" to the
 list of installed components. It is not on by default.
+> If you encountered errors with the environment above, do the following for a workaround:
+> 1.  Download and install [Build Tools for Visual Studio 2017](https://www.visualstudio.com/thank-you-downloading-visual-studio/?sku=BuildTools&rel=15)
+> 2.  Install `python2.7 x86-x64` and `virtualenv`
+> 3.  Since `mach.bat` cannot find the prompt, you should execute `Developer Command Prompt for VS 2017` manually in the Windows menu. ( You may need to choose the type, such as `x86_x64 Cross Tools Command Prompt for VS 2017`, manually if it cannot recognize type correctly. )
+> 4. `cd to/the/path/servo`
+> 5. `python mach build -d`
 
 #### Cross-compilation for Android