Added Finnish translation and improved (slightly) Contributing instructions

[minna-xD]

Happy Hacktoberfest! 🎃

I added the Finnish translation (addressing issue #279) in translations/fi.ts and updated datetime.ts and translation.ts files as instructed. Ran formatter and linter as instructed, too.

I made a small addition to the Contributing instructions because I noticed myself almost overlooking the detail. I hope you find it useful!

[IraSoro]

Hello, @minna-xD! Thank you for your interest in the project!

Thank you for adding the Finnish language and correcting the CONTRIBUTING.md file! We will review your PR soon.

[IraSoro]

@minna-xD, thank you and congratulations on your first contribution to the Peri project!

And Happy Hacktoberfest!

[minna-xD]

You're welcome! It was an interesting translation task that made me do a bit of researching on the topic :) (I deleted the branch here, I hope that's alright! Github told me it would be safe to do so and I thought it would do it in my fork, whoops!)

[IraSoro]

You're welcome! It was an interesting translation task that made me do a bit of researching on the topic :) (I deleted the branch here, I hope that's alright! Github told me it would be safe to do so and I thought it would do it in my fork, whoops!)

I'm glad you find this topic interesting! And it's okay that you deleted the branch.
Thanks again!