first-mesh comments

From Noah, here so I don't drop them:

- [ ] 1
> one thing that might be nice is a high-level view early on of what’s available to be put in this mesh. so, when it talks about adding classes, what classes are available? how does a mesh developer find this out? presumably that should be documented for any given mesh, and it should be talked about for this mesh.
“we’re going to create a mesh that consists of a broker and a service with “keyvaluestore” endpoints”
“here’s what a keyvaluestore endpoint can do . . .”

- [x] 2
> define “mesh”

- [x] 3
> the “Running and interacting” doesn’t actually mention the dl-serve command in the first paragraph
maybe a brief summary of the main executables and their functions: dl-serve to start services, dl-mon to view what messages are being sent around, and dl-agent to manually send messages