chore(l10n): update l10n readme

[skjnldsv]

Readme update:

---

Translations

This library uses translated strings.
When you edit/create a translated string, you need to run npm run l10n:extract to update the source files.
Our awesome translation community will then be notified and a bot will sync those changes automatically.

Nonetheless, it requires a bit of caution.
When you implement a translated string, import the translate or translatePlural and add it in your methods like so:

<template>
	<element>
 		{{ t('Choose') }} 
	</element>
</template>

<script>
import { translate as t } from '@nextcloud/l10n'

export default {
	methods: {
		t,
	},
}
</script>

Please note that using a translated string as an attribute will NOT work.
But it will work if it's within an element (like the example above)

<template>
	<element :prop="t('This will not work')" />
</template>

You will instead have to define the string in the data section and use the relevant variable reference.

<template>
	<element :prop="chooseProp" />
</template>

<script>
export default {
	data() {
		return {
			chooseProp: t('Choose'),
		},
	}
}
</script>