Update plugins.md on how to bundle a plugin.

[luckydonald]

Description of Problem

There exists instructions how to write a plugin,
but not how to prepare or build said plugin for npm.

Proposed Solution

Add link to vite's docs.
I don't think that's a good solution, but it's at least better than what we have now (which is none).

In the long term a detailed instruction would be great,
even better would be if we could have a scaffolding for it, see vuejs/create-vue#565

Additional Information

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[NataliaTepluhina]

Thank you for your contribution! We need to keep the newline between the heading and main text

[edimitchel]

It misses the anchor (which are all needed for translations anchor link constituency)