[WIP] R 3.4 recommendations + Benchmark/Parameter Notes

[Laurae2]

Changes:

• R users: recommend switching to CXX11 flag when R>=3.4 because it cannot be modified easily using package Makevars
• R users: recommend switching gcc version when appropriate
• R users: recommend choosing -O3 -mtune=native for maximum performance
• Some benchmarks in docs
• Links to static and interactive benchmarks
• Some pictures to add in dummy issues in GitHub

To add:

• Detailed parameters in docs

Overrides:

• Pull request Update README.md #480 (will be outdated after this PR)
• Issue [R-package] Provide recommendation for mnative? #348 (to close after this)