Update README.md

[MasonProtter]

I know there's not anything particularly new here, but I thought it'd be good to show an example where you construct an OffsetArray from a preexisting array instead of an undef. This might be redundant for experienced users but I think it's helpful for newcomers. The relativistic notation was just the first example that came to mind where 0 based indexing was natural.

It can be changed to something more natural or accessible if needed.

[timholy]

Hmm, yes, the current README is indeed pretty anemic. 👍

Why do you insert :t in the constructor instead of prior? (Just thinking we might as well make this pretty simple.) Also, since I imagine most people will be interested in numeric arrays, what about something like

η = 0.1
v1 = [sinh(η), cosh(η), 0, 0]
v0 = OffsetArray(v1, 0:3)

just to separate the values of the array from setting the indexing?

I don't feel attached to these suggestions, just wondering. And your version might have some advantages, so I'd be happy to merge this as-is.

[MasonProtter]

I did [:t, [:x, :y, :z]...] because I thought might highlight that I’m interested in a spatial, one-based vector r = [:x, :y, :z] and a zero-based spacetime vector x. I like this because it emphasizes that 0-based and 1-based arrays might be useful for two related quantities in one problem.

Perhaps it’d e good to have the symbolic example as well as the numeric example? Or I can just switch to your numeric example.

Would you still prefer that it be something like OffsetVector([:t, :x, :y, :z], 0:3) instead?

[MasonProtter]

I've added an exmple where one may want to express the coefficients of the Laurent polynomial

6/x + 5 - 2*x + 3*x^2 + x^3

as a -1-based array, since if one counts from -1, the nth coefficient corresponds to the nth power of x. This also introduces the eachindex function which is very useful when working with any AbstractArray and should be advertised to potential users of OffsetArrays.jl

[timholy]

Test failures addressed in #62

[timholy]

Thanks for an excellent and much-needed enhancement!