Update CentOS insutructions in dependencies.md 
Update CentOS insutructions in dependencies.md 

Created section for CentOS 8 instructions as it deviates from CentOS 7

- There is no python binary that can be installed with a package so it must be symlinked
- Removed packages not needed from step 4
- Removed step concerning g++ as the version in CentOS 8 is usable
- Improved wording for Steps summary in both CentOS 8 and 7
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