Added readme for Croatian language

[4dls]

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[das-g]

Dear @4dls

Thank you for this effort, but please note that we don't create translations directly in the GitHub repo, but rather on Crowdin. (See README.md).

Translating on Crowdin rather than on GitHub has several advantages:

• We can incrementally work on translations (sentence by sentence) without publishing incomplete translations.
• We can see how complete / incomplete each translation is and which sentences are yet to be translated.
• When the original changes after translations have been submitted, we can see which sentences need to be re-translated for being up-to-date.
• Several translations of the same sentence can be suggested and users can vote on the best translation.
• We can track and see which translations have been proofread and which remain to be proofread.

If you would like to contribute to the Croatian translation of the Django Girls tutorial, please do so at https://crowdin.com/project/django-girls-tutorial/hr. We'd appreciate any help we can get there.

Please do not open Pull Requests for integrating incomplete translations. Thank you.