Add README.md to all modules in SpacetimeDB/modules describing their usage

[bfops]

In response to #432 .

Now that our code is public, it's important for outside readers to be able to understand "what is this module for?" In the case above, it seems pretty obvious that the module in question is used for testing, but many of the other modules are less obvious:

• spacetimedb-quickstart is named very similarly to quickstart-chat, but the two are unrelated.
• The sdk-test* modules should describe what tests call them and what SpacetimeDB features they verify.

Also, rust-wasm-test doesn't describe in enough detail what tests use the module or what it verifies.

A simple README.md in each subdir should include:

• Whether the module is a test or an example.
• For examples, how to run it, and a link to the document describing it, if any.
• For tests, which test invokes it, and what that test is intended to verify.