README.linux.md: SCL repository must be installed before devtoolset-6 packages
- If you want to install and use gcc-6 by default, run:
```bash
- sudo yum install -y centos-release-scl devtoolset-6-toolchain
+ sudo yum install -y centos-release-scl
+ sudo yum install -y devtoolset-6-toolchain
echo 'source scl_source enable devtoolset-6' >> ~/.bashrc
```
