Create chat_effects.md

[daniellekirkwood]

Fixes #190

We discussed in the Product Team internal the need for clarity on Chat Effects. As this is product strategy that impacts the community and those wishing to contribute to our codebase, it makes sense to create documentation in Element-Meta where we can track, approve, and update any strategy or product decisions such as this...

New document here: https://github.com/vector-im/element-meta/blob/daniellekirkwood-patch-190/docs/chat_effects.md

Also added a docs/ folder to Element-Meta repo.

[daniellekirkwood]

@nadonomy & @kittykat there's 2 reviews here...

1. These are my thoughts on Chat Effects and the stage we're in with them right now. What are your thoughts on this? Anything you'd change?
2. How do you feel about keeping community relevant strategy and product documentation stored this way? I'm personally enjoying the "GitHub is a source of truth" principle and using Markdown and a ./docs/ folder allows us to do a product review before publish (unlike wiki pages) but keeps all our thinking in the open ready for community contribution against product values and ideas... Thoughts... :D

[kittykat]

I think this is worth trying out!

[daniellekirkwood]

@nadonomy I'm going to go ahead and merge this change as @kittykat has reviewed and we've talked about this on Element. Thanks