Add notes on getting p2p up and running

Partly so I don't forget myself!

p2p.md

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+## Peer-to-peer
+
+These are the instructions for setting up P2P Dendrite, current as of March 2020. You need a dendrite and a riot-web checked out.
+In addition:
+
+```
+$ git clone git@github.com:matrix-org/go-http-js-libp2p.git
+$ git clone git@github.com:matrix-org/go-sqlite3-js.git
+```
+
+Make sure to `yarn install` in both of these repos.
+
+
+### Dendrite
+
+- `kegan/wasm` branch will do.
+- Build it and copy assets to riot-web.
+```
+$ GOOS=js GOARCH=wasm go build -o main.wasm ./cmd/dendritejs
+$ cp main.wasm ../riot-web/src/vector/dendrite.wasm
+```
+
+### Rendezvous
+
+- This is how the peers discover each other.
+```
+$ npm install --global libp2p-websocket-star-rendezvous
+$ rendezvous --port=9090 --host=127.0.0.1
+```
+
+### Riot-web
+
+- `$ cp "$(go env GOROOT)/misc/wasm/wasm_exec.js" ./src/vector/`
+- Comment out the lines in `wasm_exec.js` which contains:
+```
+if (!global.fs && global.require) {
+    global.fs = require("fs");
+}
+```
+- Add the diff at https://github.com/vector-im/riot-web/compare/matthew/p2p?expand=1 - ignore the `package.json` stuff.
+- Add the following symlinks: they HAVE to be symlinks as the diff in `webpack.config.js` references specific paths.
+```
+$ cd node_modules
+$ ln -s ../../go-sqlite-js # NB: NOT go-sqlite3-js
+$ ln -s ../../go-http-js-libp2p
+```
+
+NB: If you don't run the server with `yarn start` you need to make sure your server is sending the header `Service-Worker-Allowed: /`.
+
+## Running
+
+You need a Chrome and a Firefox running to test locally as service workers don't work in incognito tabs.
+- For Chrome, use `chrome://serviceworker-internals/` to unregister/see logs.
+- For Firefox, use `about:debugging#/runtime/this-firefox` to unregister. Use the console window to see logs.
+
+Assuming you've `yarn start`ed Riot-Web, go to `http://localhost:8080` and wait a bit. Then refresh the page (this is required
+because the fetch interceptor races with setting up dendrite. If you don't refresh, you won't be able to contact your HS). After
+the refresh, click Register and use `http://localhost:8080` as your HS URL.
+
+You can join rooms by room alias e.g `/join #foo:bar`.
+
+### Known issues
+
+- When registering you may be unable to find the server, it'll seem flakey. This happens because the SW, particularly in Firefox,
+  gets killed after 30s of inactivity. When you are not registered, you aren't doing `/sync` calls to keep the SW alive, so if you
+  don't register for a while and idle on the page, the HS will disappear. To fix, unregister the SW, and then refresh the page *twice*.
+
+- The libp2p layer has rate limits, so frequent Federation traffic may cause the connection to drop and messages to not be transferred.
+  I guess in other words, don't send too much traffic?
+