Improve inline documentation of select to include examples of multiple columns not to be included

[ppalmes]

• provide examples of select(df, Not([:col1, :col2, ...])) to have multiple columns excluded

[bkamins]

We have a very similar example in https://github.com/JuliaData/DataFrames.jl/blob/master/docs/src/man/getting_started.md already (with a regular expression not a vector, but this is the same).

Also Not is already documented in InvertedIndices.jl.

Still, if you think it would be useful to make this specific example - just please propose a PR. Probably it best fits https://github.com/JuliaData/DataFrames.jl/blob/master/docs/src/man/getting_started.md.

[bkamins]

This is covered in basics.md