gn build: Clean up README.rst a bit

- Make introduction a bit shorter
- Add a `git clone` step to Quick start
- Put command to run first in each of the Quick start steps
- Use ``code`` instead of `label` throughout; this is .rst not .md

Differential Revision: https://reviews.llvm.org/D59600

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356885 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed