[DOC] improvements to "invitation to contribute" paragraph in documentation

[abhisek7154]

I have changed few lines to make it more grammatically correct and more appealing . Hope it helps.

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[fkiraly]

Thanks!

Let's go your with your new lines 17 and 18?
Line 17 and 18 do sound more friendly, thanks!

Line 16 indeed sounded strange prior to this PR, but I feel we're not quite there yet. How about:
"sktime is an open, community-driven project, seeking to provide opportunities to all. Therefore, we invite and wamly welcome any kind of contribution (not just code)."

[fkiraly]

I made some minor changes to phrasing along the lines of my suggestions - thank you for your contribution!