servo: Merge #19701 - Update Arch Linux setup to include clang (from ntenczar:update-readme-clang); r=jdm

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`clang` is needed to compile servo, but isn't listed as one of the dependencies needed for arch and isn't part of the `base-devel` group.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X]  no applicable github issue

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's a README change

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Source-Repo: https://github.com/servo/servo
Source-Revision: dd27fbcb3f31ae7a72d5d3ba5d22db318249acbd

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 52f76145122ae13d8154f1306d850fcbc35bf9f3

servo/README.md

@@ -107,7 +107,7 @@ sudo zypper install libX11-devel libexpat-devel libbz2-devel Mesa-libEGL-devel M
 #### On Arch Linux
 
 ``` sh
-sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip mesa cmake bzip2 libxmu glu pkg-config
+sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip mesa cmake bzip2 libxmu glu pkg-config clang
 ```
 #### On Gentoo Linux
 
@@ -138,9 +138,9 @@ list of installed components. It is not on by default. Visual Studio 2017 MUST i
 > 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.  Run `mach.bat build -d`. 
+> 3.  Run `mach.bat build -d`.
 
->If you have troubles with `x64 type` prompt as `mach.bat` set by default:  
+>If you have troubles with `x64 type` prompt as `mach.bat` set by default:
 > 1. you may need to choose and launch the type manually, such as `x86_x64 Cross Tools Command Prompt for VS 2017` in the Windows menu.)
 > 2. `cd to/the/path/servo`
 > 3. `python mach build -d`