Rollup merge of #51123 - akoserwal:master, r=Mark-Simulacrum

Update build instructions

It get stuck at the cloning step.
`./x.py build `
Updating only changed submodules
Updating submodule src/llvm
Submodule 'src/llvm' (https://github.com/rust-lang/llvm.git) registered for path 'src/llvm'
Cloning into '/home/username/rust/src/llvm'...
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Guillaume Gomez 2018-05-31 22:17:10 +02:00 committed by GitHub
commit 2f0e3d3046
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@ -38,6 +38,7 @@ Read ["Installation"] from [The Book].
3. Build and install:
```sh
$ git submodule update --init --recursive --progress
$ ./x.py build && sudo ./x.py install
```