Where to get a basic introduction to a science subject published legitimately? I have a high school to undergraduate level introduction written for methods and applications of a particular science topic (~5000 words with tables and schematic, like a book chapter). 

My goal is to get people especially students interested in this particular topic. My homepage does not have much traffic to help in this. I already have an offer to write a book but that will need some free time which I do not have at this moment. 

Where to get it published legitimately? Online ok ISSN or similar required. Any suggestion where this question can be legitimately asked (if not here) is appreciated.

Paid publishers please ignore.
5000 words is too few for a commercial publisher to publish as a book, and it sounds too elementary to be published in a journal.

Your best chance is likely to find a paleoclimate textbook author and offer to collaborate with him or her. They might be willing to insert your chapter into their book.