Improve documentation for `DataFrame.concat` in relation to index about data-forge-ts

[ashleydavis]

I don't think the duplicated index is a problem, in fact I rely on the index being preserved especially when the index is a date - I don't want to lose the date attached to each row.

Feel free to add a warning to the documentation.

[ashleydavis]

Please feel free to make some improvements to the docs and submit a PR!

[mz8i]

OK, will do!
Regarding the concatenation behaviour, what do you think about the current behaviour when concatenating two zero-based indices? Should it be left as-is? Are there any use cases for a dataframe with duplicate index values, or should it be considered invalid?
I'm wondering if the behaviour should at least be that a warning is printed if the index contains duplicates after the concatenation. And possibly the index gets automatically reset in such a case?

[mz8i]

I saw you closed this recently @ashleydavis - sorry for being slow about submitting a PR, I just took a stab at the wording in #164

[ashleydavis]

Thanks so much for your contribution! I really appreciate it.