Add readme for dendrite-demo-pinecone (#2851)

cmd/dendrite-demo-pinecone/README.md

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+# Pinecone Demo
+
+This is the Dendrite Pinecone demo! It's easy to get started.
+
+To run the homeserver, start at the root of the Dendrite repository and run:
+
+```
+go run ./cmd/dendrite-demo-pinecone
+```
+
+To connect to the static Pinecone peer used by the mobile demos run:
+
+```
+go run ./cmd/dendrite-demo-pinecone -peer wss://pinecone.matrix.org/public
+```
+
+The following command line arguments are accepted:
+
+* `-peer tcp://a.b.c.d:e` to specify a static Pinecone peer to connect to - you will need to supply this if you do not have another Pinecone node on your network
+* `-port 12345` to specify a port to listen on for client connections
+
+Then point your favourite Matrix client to  the homeserver URL`http://localhost:8008` (or whichever `-port` you specified), create an account and log in.
+
+If your peering connection is operational then you should see a `Connected TCP:` line in the log output. If not then try a different peer.
+
+Once logged in, you should be able to open the room directory or join a room by its ID.