Add a contributing guide in the README

[jakobnissen]

So, this is the result of some yak shaving:

1. My PR Implement StringPairs for efficient pairs(::AbstractString) julia#51631 fails the StyledString tests. This is going to be fixed on the Julia side in #51631, but this ought to also be fixed on the StyledStrings side. See Use a function over a closure to handle poor inference #62 for details.
2. However, the master branch of StyledStrings doesn't work with Julia master. Hence, I can't run the tests to see if Use a function over a closure to handle poor inference #62 actually works. This makes it difficult to contribute to this package, because it's unclear what needs to be done to get the package in a functional state to begin with.

[jakobnissen]

My bad, I was working on a stale clone of this repo. The real main branch does pass on Julia's master!