Setup PkgBenchmark

[mforets]

• Continuation of https://github.com/JuliaReach/BernsteinExpansions.jl/blob/master/benchmark/benchmarks.jl
• Adding the usage instructions in the docs
• (?) Execute benchmarks and print them on the docs each time they are generated. Or find another way for web-based reporting of the benchs results (like Python's airspeed velocity)

[mforets]

Also relevant: Alexandre's cellar of polynomials and Berstein's coeffs calculations:
https://github.com/roccaa/FPBern/tree/master/FPBern(a)/bin

It shouldn't be hard to parse these files and setup a comparison with the C++ implementation.

[mforets]

polynomials and Berstein's coeffs calculations ...

This is now #7.

Execute benchmarks and print them on the docs each time they are generated

Seems to me that this is not yet available in Julia, see for example this thread.

Adding the usage instructions in the docs

This is now #8.