[Documentation] Documentation for adding holidays

[inson1]

Checklist

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Feature description

How to add them? If they are recurring, what first date should be added? Should I delete the old ones, not reccuring? etc.

Why do you want this feature?

We need add holidays from community

Additional information

Even with added https://github.com/FossifyOrg/Calendar/discussions/11 there may some holidays be missing etc. But Please do not add just link (if it isnt official instructions)

[ghhhpm]

Please also document how to remove holidays from the calendar again. I have accidentally added holidays of another country and wanted to remove those again. I couldn't figure out how to do that, it might even not be possible?

[curbengh]

public holiday calendars are placed in app/src/main/assets/. You will need to be familiar with iCal syntax. australia.ics is a good start as it includes recurring holidays with similar dates annually and also pattern (e.g. first monday of march).

when adding recurring ones, put future dates, though current year is also fine. when updating the calendar, replace the past ones with upcoming.