Introducing Space readmes

[lostintangent]

This PR introduces the notion of a Readme to a space. This allows founders to capture whatever materials they need (e.g. links, images, docs) to ensure that new members can be efficiently on-boarded to the space. The readme is stored as a new property on the space, and exposed via a VS Code FileSystemProvider. This way, the editing/previewing experience is exactly the same as any other markdown file. However, the contents are stored entirely on the server as opposed to there 👍

[RandomFractals]

just an fyi: I like how you've implemented this feature with a simple readme. very clever and simple to use idea. very cool!