Add article on principles for creating general-purpose components

I wrote these principles for the [Basic Web Components](https://github.com/basic-web-components/components-dev/wiki) project, but they're applicable to anyone creating general-purpose components for a broad audience. These principles are still evolving, but they've reached a point where I think they're worth sharing.

**Note**: When I build the site and check the formatting of the card on the Articles page, the line spacing looks different than for other articles. In the debugger, it appears that other article cards put the description in a paragraph tag inside a div, but in the card for the new article I'm submitting, the text is directly inside the div (no paragraph tag). It's the paragraph that sets the line-height, which is why the card looks off. However, I can't see any reason why the formatting on this article would be different than others — I started by copying the contents of one of the existing .md files. Please fix or let me know how to fix.